Learning Content Selection Rules for Generating Object Descriptions in Dialogue

P. W. Jordan and M. A. Walker

Volume 24, 2005

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A fundamental requirement of any task-oriented dialogue system is the ability to generate object descriptions that refer to objects in the task domain. The subproblem of content selection for object descriptions in task-oriented dialogue has been the focus of much previous work and a large number of models have been proposed. In this paper, we use the annotated COCONUT corpus of task-oriented design dialogues to develop feature sets based on Dale and Reiter's (1995) incremental model, Brennan and Clark's (1996) conceptual pact model, and Jordan's (2000b) intentional influences model, and use these feature sets in a machine learning experiment to automatically learn a model of content selection for object descriptions. Since Dale and Reiter's model requires a representation of discourse structure, the corpus annotations are used to derive a representation based on Grosz and Sidner's (1986) theory of the intentional structure of discourse, as well as two very simple representations of discourse structure based purely on recency. We then apply the rule-induction program RIPPER to train and test the content selection component of an object description generator on a set of 393 object descriptions from the corpus. To our knowledge, this is the first reported experiment of a trainable content selection component for object description generation in dialogue. Three separate content selection models that are based on the three theoretical models, all independently achieve accuracies significantly above the majority class baseline (17%) on unseen test data, with the intentional influences model (42.4%) performing significantly better than either the incremental model (30.4%) or the conceptual pact model (28.9%). But the best performing models combine all the feature sets, achieving accuracies near 60%. Surprisingly, a simple recency-based representation of discourse structure does as well as one based on intentional structure. To our knowledge, this is also the first empirical comparison of a representation of Grosz and Sidner's model of discourse structure with a simpler model for any generation task.

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Journal of Artificial Intelligence Research 24 (2005) 157-194 Submitted 09/04; published 07/05 Learning Content Selection Rules for Generating Object Descriptions in Dialogue Pamela W. Jordan pjordan@pitt.edu Learning Research and Development Center & Intelligent Systems ProgramUniversity of Pittsburgh, LRDC Rm 744Pittsburgh, PA 15260 Marilyn A. Walker M.A.Walker@sheffield.ac.uk Department of Computer Science, University of Sheffield Regent Court, 211 Portobello Street Sheffield S1 4DP, U.K. Abstract A fundamental requirement of any task-oriented dialogue system is the ability to gen- erate object descriptions that refer to objects in the task domain. The subproblem ofcontent selection for object descriptions in task-oriented dialogue has been the focus ofmuch previous work and a large number of models have been proposed. In this paper, weuse the annotated coconut corpus of task-oriented design dialogues to develop featuresets based on Dale and Reiter’s (1995) incremental model, Brennan and Clark’s (1996)conceptual pact model, and Jordan’s (2000b) intentional influences model, and use thesefeature sets in a machine learning experiment to automatically learn a model of contentselection for object descriptions. Since Dale and Reiter’s model requires a representationof discourse structure, the corpus annotations are used to derive a representation based onGrosz and Sidner’s (1986) theory of the intentional structure of discourse, as well as twovery simple representations of discourse structure based purely on recency. We then applythe rule-induction program ripper to train and test the content selection component of anobject description generator on a set of 393 object descriptions from the corpus. To ourknowledge, this is the first reported experiment of a trainable content selection componentfor object description generation in dialogue. Three separate content selection models thatare based on the three theoretical models, all independently achieve accuracies significantlyabove the majority class baseline (17%) on unseen test data, with the intentional in-fluences model (42.4%) performing significantly better than either the incremental model(30.4%) or the conceptual pact model (28.9%). But the best performing models combineall the feature sets, achieving accuracies near 60%. Surprisingly, a simple recency-basedrepresentation of discourse structure does as well as one based on intentional structure. Toour knowledge, this is also the first empirical comparison of a representation of Grosz andSidner’s model of discourse structure with a simpler model for any generation task. 1. Introduction A fundamental requirement of any task-oriented dialogue system is the ability to generateobject descriptions that refer to objects in the task domain. For example, consider theexcerpt of a task-oriented dialogue from the coconut corpus in Figure 1 (Di Eugenio,Jordan, Thomason, & Moore, 2000). The conversants in this dialogue are attempting to c 2005 AI Access Foundation. All rights reserved.

Jordan & Walker (Partial solution to problem already agreed upon in prior dialogue: 3 green chairs for 50, 1 greenchair for 100 and a green table in the dining room, and a blue sofa in the living room. The numbersin brackets indicate the utterance breaks for analysis of the dialogue.)G: [36] That leaves us with 250 dollars. [37] I have a yellow rug for 150 dollars. [38] Do you haveany other furniture left that matches for 100 dollars?”S: [39] No, I have no furniture left that costs 100. [40] I guess you can buy the yellow rug for 150.G: [41] Okay. [42] I’ll buy the rug for 150 dollars. [43] I have a green chair [44] that I can buy for100 dollars [45] that should leave us with no money.S: [46] That sounds good. [47] Go ahead and buy the yellow rug and the green chair.G: [48] I’ll buy the green 100 dollar chair. [49] Design Complete?S: [50] Sounds good, [51] do you want the green chair in the dining room with the other chairs? [52]I put the yellow rug in the living room. [53] Then the design is complete.G: [54] Sounds good. [55] Hit the design complete Figure 1: Excerpt of a coconut dialogue illustrating variable selection of attributes for object descriptions that refer to furniture items collaboratively construct a solution for furnishing a two room house. Each conversant startsthe task with a separate set of furniture items that can be used in the solution. In the processof negotiating the solution, they generate object descriptions (shown in italics) for items offurniture. Each furniture type in the coconut task domain has four associated attributes: color, price, owner and quantity. As a first step, an object description generator must decide whichof these four attributes to include in an utterance, while subsequent surface generation stepsdecide where in the utterance the attributes will be expressed. For example, the task domainobjects under discussion in the dialogue in Figure 1 are a 150 yellow rug owned by Garrett(G) and a 100 dollar green chair owned by Steve (S). In the dialogue excerpt in Figure1, the yellow rug is first referenced in utterance 36 as a yellow rug for 150 dollars andthen subsequently as the yellow rug for 150 dollars, the rug for 150 dollars, the yellow rug,where the owner attribute is sometimes realized in a separate noun phrase within the sameutterance. It could also have been described by any of the following: the rug, my rug, myyellow rug, my 150 yellow rug, the 150 rug. The content of these object descriptionsvaries depending on which attributes are included. How does the speaker decide whichattributes to include? The problem of content selection for subsequent reference has been the focus of much previous work and a large number of overlapping models have been proposed that seek toexplain different aspects of referring expression content selection (Clark & Wilkes-Gibbs,1986; Brennan & Clark, 1996; Dale & Reiter, 1995; Passonneau, 1995; Jordan, 2000b) interalia. The factors that these models use include the discourse structure, the attributes andattribute values used in the previous mention, the recency of last mention, the frequency ofmention, the task structure, the inferential complexity of the task, and ways of determiningsalient objects and the salient attributes of an object. In this paper, we use a set of factorsconsidered important for three of these models, and empirically compare the utility of these 158

Learning Content Selection Rules for Generating Object Descriptions factors as predictors in a machine learning experiment in order to first establish whether theselected factors, as we represent them, can make effective contributions to the larger task ofcontent selection for initial as well as subsequent reference. The factor sets we utilize are: • contrast set factors, inspired by the incremental model of Dale and Reiter (1995); • conceptual pact factors, inspired by the models of Clark and colleagues (Clark & Wilkes-Gibbs, 1986; Brennan & Clark, 1996); • intentional influences factors, inspired by the model of Jordan (2000b). We develop features representing these factors, then use the features to represent exam- ples of object descriptions and the context in which they occur for the purpose of learninga model of content selection for object descriptions. Dale and Reiter’s incremental model focuses on the production of near-minimal sub- sequent references that allow the hearer to reliably distinguish the task object from similartask objects. Following Grosz and Sidner (1986), Dale and Reiter’s algorithm utilizes dis-course structure as an important factor in determining which objects the current objectmust be distinguished from. The model of Clark, Brennan and Wilkes-Gibbs is based onthe notion of a conceptual pact, i.e. the conversants attempt to coordinate with oneanother by establishing a conceptual pact for describing an object. Jordan’s intentionalinfluences model is based on the assumption that the underlying communicative andtask-related inferences are important factors in accounting for non-minimal descriptions.We describe these models in more detail in Section 3 and explain why we expect thesemodels to work well in combination. Many aspects of the underlying content selection models are not well-defined from an implementation point of view, so it may be necessary to experiment with different definitionsand related parameter settings to determine which will produce the best performance for amodel, as was done with the parameter setting experiments carried out by Jordan (2000b).1However, in the experiments we describe in this paper, we strive for feature representationsthat will allow the machine learner to take on more of the task of finding optimal settingsand otherwise use the results reported by Jordan (2000b) for guidance. The only variationwe test here is the representation of discourse structure for those models that require it.Otherwise, explicit tests of different interpretations of the models are left to future work. We report on a set of experiments designed to establish the predictive power of the fac- tors emphasized in the three models by using machine learning to train and test the contentselection component of an object description generator on a set of 393 object descriptionsfrom the corpus of coconut dialogues. The generator goes beyond each of the models’accounts for anaphoric expressions to address the more general problem of generating bothinitial and subsequent expressions. We provide the machine learner with distinct sets offeatures motivated by these models, in addition to discourse features motivated by assumed 1. Determining optimal parameter settings for a machine learning algorithm is a similar issue (Daelemans & Hoste, 2002) but at a different level. We use the same machine learner and parameter settings for allour experiments although searching for optimal machine learner parameter settings may be of value infurther improving performance. 159

Jordan & Walker familiarity distinctions (Prince, 1981) (i.e. new vs. evoked vs. inferable discourse entities),and dialogue specific features such as the speaker of the object description, its absolutelocation in the discourse, and the problem that the conversants are currently trying tosolve. We evaluate the object description generator by comparing its predictions againstwhat humans said at the same point in the dialogue and only counting as correct those thatexactly match the content of the human generated object descriptions (Oberlander, 1998).2This provides a rigorous test of the object description generator since in all likelihood thereare other object descriptions that would have achieved the speaker’s communicative goals. We also quantify the contribution of each feature set to the performance of the object description generator. The results indicate that the intentional influences features, theincremental features and the conceptual pact features are all independently signifi-cantly better than the majority class baseline for this task, with the intentional influ-ences model (42.4%) performing significantly better than either the incremental model(30.4%) or the conceptual pact model (28.9%). However, the best performing modelscombine features from all the models, achieving accuracies at matching human performancenear 60.0%, a large improvement over the majority class baseline of 17% in which the gen-erator simply guesses the most frequent attribute combination. Surprisingly, our resultsin experimenting with different discourse structure parameter settings show that featuresderived from a simple recency-based model of discourse structure contribute as much to thisparticular task as one based on intentional structure. The coconut dataset is small compared to those used in most machine learning ex- periments. Smaller datasets run a higher risk of overfitting and thus specific performanceresults should be interpreted with caution. In addition the coconut corpus represents onlyone type of dialogue; typed, collaborative, problem solving dialogues about constraint satis-faction problems. While the models and suggested features focus on general communicativeissues, we expect variations in the task involved and in the communication setting to im-pact the predictive power of the feature sets. For example, the conceptual pact modelwas developed using dialogues that focus on identifying novel, abstract figures. Becausethe figures are abstract it is not clear at the start of a series of exercises what descriptionwill best help the dialogue partner identify the target figure. Thus the need to negotiatea description for the figures is more prominent than in other tasks. Likewise we expectconstraint satisfaction problems and the need for joint agreement on a solution to cause the intentional influences model to be more prominent for the coconut dialogues. Butthe fact that the conceptual pact features show predictive power that is significantlybetter than the baseline suggests that while the prominence of each model inspired featureset may vary across tasks and communication settings, we expect each to have a significantcontribution to make to a content selection model. Clearly, for those of us whose ultimate goal is a general model of content selection for dialogue, we need to carry out experiments on a wide range of dialogue types. Butfor those of us whose ultimate goal is a dialogue application, one smaller corpus that isrepresentative of the anticipated dialogues is probably preferable. Despite the two notes of 2. Note that the more attributes a discourse entity has, the harder it is to achieve an exact match to a human description, i.e. for this problem the object description generator must correctly choose among16 possibilities represented by the power set of the four attributes. 160

Learning Content Selection Rules for Generating Object Descriptions caution we expect our feature representations to suggest a starting point for both largerendeavors. Previous research has applied machine learning to several problems in natural language generation, such as cue word selection (Di Eugenio, Moore, & Paolucci, 1997), accent place-ment (Hirschberg, 1993), determining the form of an object description (Poesio, 2000),content ordering (Malouf, 2000; Mellish, Knott, Oberlander, & O’Donnell, 1998; Duboue& McKeown, 2001; Ratnaparkhi, 2002), sentence planning (Walker, Rambow, & Rogati,2002), re-use of textual descriptions in automatic summarization (Radev, 1998), and sur-face realization (Langkilde & Knight, 1998; Bangalore & Rambow, 2000; Varges & Mellish,2001). The only other machine learning approaches for content selection are those of Oh and Rudnicky (2002) and of Roy (2002). Oh and Rudnicky report results for automaticallytraining a module for the CMU Communicator system that selects the attributes that thesystem should express when implicitly confirming flight information in an ongoing dialogue.For example, if the caller said I want to go to Denver on Sunday, the implicit confirmationby the system might be Flying to Denver on Sunday. They experimentally compared astatistical approach based on bigram models with a strategy that only confirms informationthat the system has just heard for the first time, and found that the two systems performedequally well. Roy reports results for a spoken language generator that is trained to generatevisual descriptions of geometric objects when provided with features of visual scenes. Roy’sresults show that the understandability of the automatically generated descriptions is only8.5% lower than human-generated descriptions. Unlike our approach, neither of these con-sider the effects of ongoing dialogue with a dialogue partner, or the effect of the dialoguecontext on the generated descriptions. Our work, and the theoretical models it is basedon, explicitly focus on the processes involved in generating descriptions and redescriptionsof objects in interactive dialogue that allow the dialogue partners to remain aligned as thedialogue progresses (Pickering & Garrod, 2004). The most relevant prior work is that of Jordan (2000b). Jordan implemented Dale and Reiter’s incremental model and developed and implemented the intentional influ-ences model, which incorporates the incremental model, and tested them both againstthe coconut corpus. Jordan also experimented with different parameter settings for vagueparts of the models. The results of this work are not directly comparable because Jordanonly tested rules for subsequent reference, while here we attempt to learn rules for gener-ating both initial and subsequent references. However, using a purely rule-based approach,the best accuracy that Jordan reported was 69.6% using a non-stringent scoring criterion(not an exact match) and 24.7% using the same stringent exact match scoring used here.In this paper, using features derived from Jordan’s corpus annotations, and applying ruleinduction to induce rules from training data, we achieve an exact match accuracy of nearly47% when comparing to the most similar model and an accuracy of nearly 60% when com-paring to the best overall model. These results appear to be an improvement over thosereported by Jordan (2000b), given both the increased accuracy and the ability to generateinitial as well as subsequent references. Section 2 describes the coconut corpus, definitions of discourse entities and object descriptions for the coconut domain, and the annotations on the corpus that we useto derive the feature sets. Section 3 presents the theoretical models of content selection 161

Jordan & Walker Opal 1 YOUR INVENTORY 1 TABLE-HIGH YELLOW 400 End of Turn 0 SOFA GREEN 350 1 SOFA YELLOW 400 1 RUG RED 200 Design Complete 1 LAMP-FLOOR BLUE 50 2 CHAIR BLUE 75 0 CHAIR GREEN 100 0 CHAIR RED 100 PARTNER’S INVENTORY TABLE-LOW TABLE-HIGH RUG SOFA LAMP-TABLE LAMP-FLOOR > change the chairs, CHAIR I have two red ones for the same price. ARMCHAIR As much as I like green DESK it looks ugly with red. LIVING-ROOM DINING-ROOM > we bought the green sofa 350,the green table 400,and 2 green chairs 100 each. 100 400 350 100 100 100 400 Your budget is: 400 Figure 2: A snapshot of the interface for the coconut task for object descriptions in more detail and describes the features inspired by the models.Section 4 describes the experimental design and Section 5 presents the quantitative resultsof testing the learned rules against the corpus, discusses the features that the machinelearner identifies as important, and provides examples of the rules that are learned. Section6 summarizes the results and discusses future work. 2. The Coconut Corpus The coconut corpus is a set of 24 computer-mediated dialogues consisting of a total of1102 utterances. The dialogues were collected in an experiment where two human subjectscollaborated on a simple design task, that of buying furniture for two rooms of a house(Di Eugenio et al., 2000). Their collaboration was carried out through a typed dialoguein a workspace where each action and utterance was automatically logged. An excerptof a coconut dialogue is in Figure 1. A snapshot of the workspace for the coconutexperiments is in Figure 2. In the experimental dialogues, the participants’ main goal is to negotiate the purchases; the items of highest priority are a sofa for the living room and a table and four chairs for thedining room. The participants also have specific secondary goals which further constrainthe problem solving task. Participants are instructed to try to meet as many of thesegoals as possible, and are motivated to do so by rewards associated with satisfied goals. 162

Learning Content Selection Rules for Generating Object Descriptions The secondary goals are: 1) match colors within a room, 2) buy as much furniture as youcan, 3) spend all your money. The participants are told which rewards are associated withachieving each goal. Each participant is given a separate budget (as shown in the mid-bottom section of Figure 2) and an inventory of furniture (as shown in the upper-left section of Figure 2).Furniture types include sofas, chairs, rugs and lamps, and the possible colors are red, green,yellow or blue. Neither participant knows what is in the other’s inventory or how muchmoney the other has. By sharing information during the conversation, they can combinetheir budgets and select furniture from each other’s inventories. Note that since a participantdoes not know what furniture his partner has available until told, there is a menu (see themid-right section of Figure 2) that allows the participant to create furniture items based onhis partner’s description of the items available. The participants are equals and purchasingdecisions are joint. In the experiment, each set of participants solved one to three scenarioswith varying inventories and budgets. The problem scenarios varied task complexity byranging from tasks where items are inexpensive and the budget is relatively large, to taskswhere the items are expensive and the budget relatively small. 2.1 Discourse Entities and Object Descriptions in the Corpus A discourse model is used to keep track of the objects discussed in a discourse. As an objectis described, the conversants relate the information about the object in the utterance to theappropriate mental representation of the object in the discourse model (Karttunen, 1976;Webber, 1978; Heim, 1983; Kamp & Reyle, 1993; Passonneau, 1996). The model containsdiscourse entities, attributes and links between entities (Prince, 1981). A discourse entityis a variable or placeholder that indexes the information about an object described in aparticular linguistic description to an appropriate mental representation of the object. Thediscourse model changes as the discourse progresses. When an object is first described, adiscourse entity such as ei is added to the discourse model. As new utterances are produced,additional discourse entities may be added to the model when new objects are described,and new attributes may get associated with ei whenever it is redescribed. Attributes are notalways supplied by a noun phrase (NP). They may arise from other parts of the utteranceor from discourse inference relations that link to other discourse entities. To illustrate the discourse inference relations relevant to coconut, in (1b) the green set is an example of a new discourse entity which has a set/subset discourse inference relationto the three distinct discourse entities for 2 25 green chairs, 2 100 green chairs and 1200 green table. (1) a. : I have [2 25 green chairs] and [a 200 green table]. b. : I have [2 100 green chairs]. Let’s get [the green set]. A class inference relation exists when the referent of a discourse entity has a subsumption relationship with a previous discourse entity. For example, in (2) the table and your greenone have a subsumption relationship. (2) Let’s decide on [the table] for the dining room. How about [your green one]? 163

Jordan & Walker A common noun anaphora inference relation occurs in the cases of one anaphora and null anaphora. For example, in (3) each of the marked NPs in the last part of the utterancehas a null anaphora relation to the marked NP in the first part. Note that this examplealso has a class inference relation as well. (3) I have [a variety of high tables] ,[green], [red] and [yellow] for 400, 300, and 200. Discourse entities can also be related by predicative relationships such as is. For exam- ple, in (4) the entities defined by my cheapest table and a blue one for 200 are not thesame discourse entities but the information about one provides more information about theother. Note that this example also includes common noun anaphora and class inferencerelations. (4) [My cheapest table] is [a blue one for 200]. An object description is any linguistic expression (usually an NP) that initiates the cre- ation or update of a discourse entity for a furniture item, along with any explicit attributesexpressed within the utterance. We consider the attributes that are explicitly expressedoutside of an NP to be part of the object description since they can be realized either aspart of the noun phrase that triggers the discourse entity or elsewhere in the utterance.Attributes that are inferred (e.g. quantity from “a” or “the”) help populate the discourseentity but are not considered part of an object description since inferred attributes mayor may not reflect an explicit choice. The inferred attribute could be a side-effect of thesurface structure selected for realizing the object description.3 2.2 Corpus Annotations After the corpus was collected, it was annotated by human coders for three types of features: problem-solving utterance level features as shown in Figure 3, discourseutterance level features as illustrated in Figure 4 and discourse entity level fea-tures as illustrated in Figure 5. Some additional features are shown in Figure 6. Each ofthe feature encodings shown are for the dialogue excerpt in Figure 1. All of the features were hand-labelled on the corpus because it is a human-human corpus but, as we will discuss further at the end of this section, many of these features would needto be established by a system for its collaborative problem solving component to functionproperly. Looking first at Figure 6, it is the explicit attributes (as described in the previous section) that are to be predicted by the models we are building and testing. The remainingfeatures are available as context for making the predictions. The problem-solving utterance level features in Figure 3 capture the problem solving state in terms of the goals and actions that are being discussed by the conversants,constraint changes that are implicitly assumed, or explicitly stated by the conversants, andthe size of the solution set for the current constraint equations. The solution set size for 3. While the same is true of some of the attributes that are explicitly expressed (e.g. “I” in subject position expresses the ownership attribute), most of the attribute types of interest in the corpus are adjuncts(e.g. “Let’s buy the chair [for 100].”). 164

Learning Content Selection Rules for Generating Object Descriptions Utterance Goal/ Introduce Goal/ Change Solution Action or Action in Size Label continue Identifier Constraints 37 SelectOptionalItemLR introduce act4 drop color match indeterminate 38 SelectOptionalItem introduce act5 color,price limit indeterminate 39 SelectOptionalItem continue act5 none indeterminate 40 SelectOptionalItemLR continue act4 none determinate 42 SelectOptionalItemLR continue act4 none determinate 43 SelectOptionalItemDR continue act5 none indeterminate 44 SelectOptionalItemDR continue act5 none determinate 46 SelectOptionalItemDR continue act5 none determinate 47 SelectOptionalItemDR continue act4 none determinate SelectOptionalItemLR continue act5 48 SelectOptionalItemDR continue act5 none determinate 51 SelectOptionalItemDR, continue act5, none determinate SelectChairs introduce act3 52 SelectOptionalItemLR continue act4 none determinate Figure 3: Problem solving utterance level annotations for utterances relevant to problem solving goals and actions for the dialogue excerpt in Figure 1 Utterance Influence Influence on Listener on Speaker 37 ActionDirective Offer 40 ActionDirective Commit 42 ActionDirective Commit 43 OpenOption nil 44 ActionDirective Offer 46 ActionDirective Commit 47 ActionDirective Commit 48 ActionDirective Commit 49 ActionDirective Offer 51 ActionDirective Offer 52 ActionDirective Commit Figure 4: Discourse utterance level annotations for utterances relevant to establishing joint agreements for the dialogue excerpt in Figure 1 a constraint equation is characterized as being determinate if the set of values is closedand represents that the conversants have shared relevant values with one another. Anindeterminate size means that the set of values in still open and so a solution cannot yetbe determined. The problem-solving features capture some of the situational or problem-solving influences that may effect descriptions and indicate the task structure from whichthe discourse structure can be derived (Terken, 1985; Grosz & Sidner, 1986). Each domain 165

Jordan & Walker Utterance Reference Discourse Attribute Argument and Inference Values for Goal/Action Coreference Relations Identifier 37 initial ref-1 nil my,1,yellow,rug,150 act4 38 initial ref-2 nil your,furniture,100 act5 39 initial ref-3 class to ref-20 my,furniture,100 act5 40 corefers ref-1 nil your,1,yellow,rug,150 act4 42 corefers ref-1 nil my,1,rug,150 act4 43 initial ref-4 nil my,1,green,chair act5 44 corefers ref-4 CNAnaphora ref-4 my,100 act5 47 corefers ref-1 nil your,1,yellow,rug act4 47 corefers ref-4 nil your,1,green,chair act5 48 corefers ref-4 nil my,1,green,chair,100 act5 51 corefers ref-4 nil 1,green,chair act5 51 initial ref-5 set of ref-12,ref-16 chair act3 52 corefers ref-1 1,yellow rug act4 Figure 5: Discourse entity level annotations for utterances referring to furniture items in Figure 1 Utterance Speaker Explicit Inferred Description Attributes Attributes 37 G type,color,price,owner quantity a yellow rug for 150 dollars 38 G type,color,price,owner furniture ... for 100 dollars 39 S type,price,owner furniture ... 100 dollars 40 S type,color,price,owner quantity the yellow rug for 150 42 G type,price,owner quantity the rug for 150 dollars 43 G type,color,owner quantity a green chair 44 G price,owner [0] for 100 dollars 47 S type,color owner,quantity the yellow rug 47 S type,color owner,quantity the green chair 48 G type,color,price,owner quantity the green 100 dollar chair 51 S type,color quantity the green chair 51 S type quantity the other chairs 52 S type,color quantity the yellow rug Figure 6: Additional features for the dialogue excerpt in Figure 1 goal provides a discourse segment purpose so that each utterance that relates to a differentdomain goal or set of domain goals defines a new segment. The discourse utterance level features in Figure 4 encode the influence the ut- terance is expected to have on the speaker and the listener as defined by the DAMSLscheme (Allen & Core, 1997). These annotations also help capture some of the situationalinfluences that may effect descriptions. The possible influences on listeners include openoptions, action directives and information requests. The possible influences on speakers areoffers and commitments. Open options are options that a speaker presents for the hearer’sfuture actions, whereas with an action directive a speaker is trying to put a hearer under 166

Learning Content Selection Rules for Generating Object Descriptions an obligation to act. There is no intent to put the hearer under obligation to act withan open option because the speaker may not have given the hearer enough information toact or the speaker may have clearly indicated that he does not endorse the action. Offersand commitments are both needed to arrive at a joint commitment to a proposed action.With an offer the speaker is conditionally committing to the action whereas with a committhe speaker is unconditionally committing. With a commit, the hearer may have alreadyconditionally committed to the action under discussion, or the speaker may not care if thehearer is also committed to the action he intends to do. The discourse entity level features in Figure 5 define the discourse entities that are in the discourse model. Discourse entities, links to earlier discourse entities and theattributes expressed previously for a discourse entity at the NP-level and utterance levelare inputs for an object description generator. Part of what is used to define the discourseentities is discourse reference relations which include initial, coreference and discourse infer-ence relations between different entities such as the links we described earlier; set/subset,class, common noun anaphora and predicative. In addition, in order to link the expressionto appropriate problem solving actions, the action for which the entity is an argument isalso annotated. In order to test whether an acceptable object description is generated bya model for a discourse entity in context, the explicit attributes used to describe the entityare also annotated (recall Figure 6). Which action an entity is related to helps associate entities with the correct parts of the discourse structure and helps determine which problem-solving situations are relevantto a particular entity. From the other discourse entity level annotations, initial represen-tations of discourse entities and updates to them can be derived. For example, the initialrepresentation for “I have a yellow rug. It costs 150.” would include type, quantity, colorand owner following the first utterance. Only the quantity attribute is inferred. After thesecond utterance the entity would be updated to include price. The encoded features all have good inter-coder reliability as shown by the kappa values given in Table 1 (Di Eugenio et al., 2000; Jordan, 2000b; Krippendorf, 1980). These valuesare all statistically significant for the size of the labelled data set, as shown by the p-valuesin the table. Discourse Reference Discourse Argument Attributes Entity and Inference for Goal/ Level Coreference Relations Action .863 .819 .857 .861 (z=19, p<.01) (z=14, p<.01) (z=16, p<.01) (z=53, p<.01) Problem Introduce Continue Change in Solution Goal/Action Solving Goal/Action Goal/Action Constraints Size Utterance .897 .857 .881 .8 .74 Level (z=8, p<.01) (z=27, p<.01) (z=11, p<.01) (z=6, p<.01) (z=12, p<.01) Discourse Influence Influence Utterance on Listener on Speaker Level .72 .72 (z=19, p<.01) (z=13, p<.01) Table 1: Kappa values for the annotation scheme 167

Jordan & Walker While the availability of some of this annotated information in a dialogue system is currently an ongoing challenge for today’s systems, a system that is to be a successfuldialogue partner in a collaborative problem solving dialogue, where all the options are notknown a priori, will have to model and update discourse entities, understand the currentproblem solving state and what has been agreed upon, and be able to make, accept or rejectproposed solutions. Certainly, not all dialogue system domains and communicative settingswill need all of this information and likewise some of the information that is essential forother domains and settings will not be necessary to engage in a coconut dialogue. The experimental data consists of 393 non-pronominal object descriptions from 13 dia- logues of the coconut corpus as well as features constructed from the annotations describedabove. The next section explains in more detail how the annotations are used to constructthe features used in training the models. 3. Representing Models of Content Selection for Object Descriptions as Features In Section 1, we described how we would use the annotations on the coconut corpus toconstruct feature sets motivated by theories of content selection for object descriptions.Here we describe these theories in more detail, and present, with each theory, the featuresets that are inspired by the theory. In Section 4 we explain how these features are used toautomatically learn a model of content selection for object descriptions. In order to be usedin this way, all of the features must be represented by continuous (numeric), set-valued, orsymbolic (categorial) values. Models of content selection for object descriptions attempt to explain what motivates a speaker to use a particular set of attributes to describe an object, both on the first mentionof an object as well as in subsequent mentions. In an extended discourse, speakers oftenredescribe objects that were introduced earlier in order to say something more about theobject or the event in which it participates. We will test in part an assumption that manyof the factors relevant for redescriptions will also be relevant for initial descriptions. All of the models described below have previously had rule-based implementations of them tested on the coconut corpus and were all found to be nearly equally good at explain-ing the redescriptions in the corpus (Jordan, 2000b). All of them share a basic assumptionabout the speaker’s goal when redescribing a discourse entity already introduced into thediscourse model in prior conversation. The speaker’s primary goal is identification, i.e. togenerate a linguistic expression that will efficiently and effectively re-evoke the appropriatediscourse entity in the hearer’s mind. A redescription must be adequate for re-evoking theentity unambiguously, and it must do so in an efficient way (Dale & Reiter, 1995). Onefactor that has a major effect on the adequacy of a redescription is the fact that a discourseentity to be described must be distinguished from other discourse entities in the discoursemodel that are currently salient. These other discourse entities are called distractors. Char-acteristics of the discourse entities evoked by the dialogue such as recency and frequencyof mention, relationship to the task goals, and position relative to the structure of thediscourse are hypothesized as means of determining which entities are mutually salient forboth conversants. 168

Learning Content Selection Rules for Generating Object Descriptions • what is mutually known: type-mk, color-mk, owner-mk, price-mk, quantity-mk • reference-relation: one of initial, coref, set, class, cnanaphora, predicative Figure 7: Assumed Familiarity Feature Set. We begin the encoding of features for the object description generator with features representing the fundamental aspects of a discourse entity in a discourse model. We dividethese features into two sets: the assumed familiarity feature set and the inherentfeature set. The assumed familiarity features in Figure 7 encode all the informationabout a discourse entity that is already represented in the discourse model at the pointin the discourse at which the entity is to be described. These attributes are assumed tobe mutually known by the conversational participants and are represented by five booleanfeatures: type-mk, color-mk, owner-mk, price-mk, quantity-mk. For example, if type-mk hasthe value of yes, this represents that the type attribute of the entity to be described ismutually known. Figure 7 also enumerates a reference-relation feature as described in Section 2 to encode whether the entity is new (initial), evoked (coref) or inferred relative to the discoursecontext. The types of inferences supported by the annotation are set/subset, class, commonnoun anaphora (e.g. one and null anaphora), and predicative (Jordan, 2000b), which arerepresented by the values (set,class,cnanaphora,predicative). These reference rela-tions are relevant to both initial and subsequent descriptions. • utterance-number, speaker-pair, speaker, problem-number • attribute values: – type: one of sofa, chair, table, rug, lamp, superordinate – color: one of red, blue, green, yellow – owner: one of self, other, ours – price: range from 50 to 600 – quantity: range from 0 to 4. Figure 8: Inherent Feature Set: Task, Speaker and Discourse Entity Specific features. The inherent features in Figure 8 are a specific encoding of particulars about the discourse situation, such as the speaker, the task, and the actual values of the entity’s knownattributes (type, color, owner, price, quantity). We supply the values for the attributes incase there are preferences associated with particular values. For example, there may be apreference to include quantity, when describing a set of chairs, or price, when it is high. The inherent features allow us to examine whether there are individual differences in selection models (speaker, speaker-pair), or whether specifics about the attributes of the 169

Jordan & Walker object, the location within the dialogue (utterance-number), and the problem difficulty(problem-number) play significant roles in selecting attributes. The attribute values for anentity are derived from annotated attribute features and the reference relations. We don’t expect rules involving this feature set to generalize well to other dialogue situations. Instead we expect them to lead to a situation specific model. Wheneverthese features are used there is overfitting regardless of the training set size. Consider thata particular speaker, speaker-pair or utterance number are specific to particular dialoguesand are unlikely to occur in another dialogue, even a new coconut dialogue. These featurerepresentations would have to be abstracted to be of value in a generator. 3.1 Dale and Reiter’s Incremental Model Most computational work on generating object descriptions for subsequent reference (Ap-pelt, 1985a; Kronfeld, 1986; Reiter, 1990; Dale, 1992; Heeman & Hirst, 1995; Lochbaum,1995; Passonneau, 1996; van Deemter, 2002; Gardent, 2002; Krahmer, van Erk, & Verleg,2003) concentrates on how to produce a minimally complex expression that singles outthe discourse entity from a set of distractors. The set of contextually salient distractors isidentified via a model of discourse structure as mentioned above. Dale and Reiter’s incre-mental model is the basis of much of the current work that relies on discourse structureto determine the content of object descriptions for subsequent reference. The most commonly used account of discourse structure for task-oriented dialogues is Grosz and Sidner’s (1986) theory of the attentional and intentional structure of discourse.In this theory, a data structure called a focus space keeps track of the discourse entitiesthat are salient in a particular context, and a stack of focus spaces is used to store the focusspaces for the discourse as a whole. The content of a focus space and operations on thestack of focus spaces is determined by the structure of the task. A change in task or topicindicates the start of a new discourse segment and a corresponding focus space. All of thediscourse entities described in a discourse segment are classified as salient for the dialogueparticipants while the corresponding focus space is on the focus stack. Approaches that usea notion of discourse structure take advantage of this representation to produce descriptorsthat are minimally complex given the current focus space, i.e. the description does not haveto be unambiguous with respect to the global discourse. According to Dale and Reiter’s model, a descriptor containing information that is not needed to identify the referent given the current focus space would not be minimally complexbut a small number of overspecifications that appear relative to the identification goal areexpected and can be explained as artifacts of cognitive processing limits. Trying to producea minimally complex description can be seen as an implementation of the two parts ofGrice’s Maxim of Quantity, according to which an utterance should both say as much asis required, and no more than is required (Grice, 1975). Given an entity to describe and adistractor set defined by the entities in the current focus space, the incremental modelincrementally builds a description by checking a static ordering of attribute types andselecting an attribute to include in the description if and only if it eliminates some of theremaining distractors. As distractors are ruled out, they no longer influence the selectionprocess. 170

Learning Content Selection Rules for Generating Object Descriptions • Distractor Frequencies: type-distractors, color-distractors, owner-distractors, price-distrac- tors, quantity-distractors • Attribute Saliency: majority-type, majority-type-freq, majority-color, majority-color-freq, majority-price, majority-price-freq, majority-owner, majority-owner-freq, majority-quantity,majority-quantity-freq Figure 9: contrast set Feature Sets A set of features called contrast set features are used to represent aspects of Dale and Reiter’s model. See Figure 9. The goal of the encoding is to represent whether thereare distractors present in the focus space which might motivate the inclusion of a particularattribute. First, the distractor frequencies encode how many distractors have an attributevalue that is different from that of the entity to be described. The incremental model also utilizes a preferred salience ordering for attributes and eliminates distractors as attributes are added to a description. For example, adding theattribute type when the object is a chair, eliminates any distractors that aren’t chairs. Afeature based encoding cannot easily represent a distractor set that changes as attributechoices are made. To compensate, our encoding treats attributes instead of objects asdistractors so that the attribute saliency features encode which attribute values are mostsalient for each attribute type, and a count of the number of distractors with this attributevalue. For example, if 5 of 8 distractors are red then majority-color is red and the majority-color-freq is 5. Taking the view of attributes as distractors has the advantage that thepreferred ordering of attributes can adjust according to the focus space. This interpretationof Dale and Reiter’s model was shown to be statistically similar to the strict model butwith a higher mean match to the corpus (Jordan, 2000b). Thus our goal in adding theseadditional features is to try to obtain the best possible performance for the incrementalmodel. Finally, an open issue with deriving the distractors is how to define a focus space (Walker, 1996a). As described above, Grosz and Sidner’s theory of discourse creates a data structurecalled a focus space for each discourse segment, where discourse segments are based on theintentions underlying the dialogue. However Grosz and Sidner provide no clear criterionfor assigning the segmentation structure. In order to explore what definition variations willwork best, we experiment with three focus space definitions, two very simple focus spacedefinitions based on recency, and the other based on intentional structure as describedbelow. To train and test for the three focus space definitions, we create separate datasetsfor each of the three. To our knowledge, this is the first empirical comparison of Grosz andSidner’s model with a simpler model for any discourse-related task. For intentional structure, we utilize the problem solving utterance features hand-labelled on the coconut corpus with high reliability as discussed above in Section 2. The annotatedtask goals are used to derive an intentional structure for the discourse, which provides asegmentation of the discourse, as described by Grosz and Sidner (1986). The current focusspace as defined by the annotated task goals is used to define segment distractors. Thisdataset we label as segment. For recency, one extremely simple focus space definition 171

Jordan & Walker uses only the discourse entities from the most recent utterance as possible distractors. Thisdataset we label as one utterance. The second extremely simple focus space definitiononly considers the discourse entities from the last five utterances as possible distractors.This dataset we label as five utterance. For each dataset, the features in Figure 9 arecomputed relative to the distractors determined by its focus space definition. 3.2 Jordan’s Intentional Influences Model Jordan (2000b) proposed a model to select attributes for object descriptions for subse-quent reference called the intentional influences model. This model posits that alongwith the identification goal, task-related inferences and the agreement process for task ne-gotiation are important factors in selecting attributes. Attributes that are not necessaryfor identification purposes may be intentional redundancies with a communicative purpose(Walker, 1996b) and not always just due to cognitive processing limits on finding minimallycomplex descriptions (Jordan, 2000b). A goal-directed view of sentence generation suggests that speakers can attempt to satisfy multiple goals with each utterance (Appelt, 1985b). It suggests that this strategy alsoapplies to lower-level forms within the utterance (Stone & Webber, 1998). That is, the sameform can opportunistically contribute to the satisfaction of multiple goals. This many-onemapping of goals to linguistic forms is more generally referred to as overloading intentions(Pollack, 1991). Subsequent work has shown that this overloading can involve trade-offsacross linguistic levels. That is, an intention which is achieved by complicating a form atone level may allow the speaker to simplify another level by omitting important information.For example, a choice of clausal connectives at the pragmatic level can simplify the syntacticlevel (Di Eugenio & Webber, 1996), and there are trade-offs in word choice at the syntaxand semantics levels (Stone & Webber, 1998). The intentional influences model incorporates multiple communicative and prob- lem solving goals in addition to the main identification goal in which the speaker intendsthe hearer to re-evoke a particular discourse entity. The contribution of this model is that itoverloads multiple, general communicative and problem solving goals when generating a de-scription. When the model was tested on the coconut corpus, inferences about changes inthe problem solving constraints, about conditional and unconditional commitments to pro-posals, and about the closing of goals were all shown to be relevant influences on attributeselection (Jordan, 2000a, 2002) while goals to verify understanding and infer informationalrelations were not (Jordan, 2000b).4 The features used to approximate Jordan’s model are in Figure 10. These features cover all of the general communicative and problem solving goals hypothesized by the modelexcept for the identification goal and the information relation goal. Because of the difficultyof modelling an information relation goal with features, its representation is left to futurework.5 4. A different subset of the general goals covered by the model are expected to be influential for other domains and communication settings, therefore a general object description generator would need to betrained on a wide range of corpora. 5. Information relation goals may relate two arbitrarily distant utterances and additional details beyond distance are expected to be important. Because this goal previously did not appear relevant for the coconut corpus (Jordan, 2000b), we gave it a low priority for implementation. 172

Learning Content Selection Rules for Generating Object Descriptions • task situation: goal, colormatch, colormatch-constraintpresence, pricelimit, pricelimit-con- straintpresence, priceevaluator, priceevaluator-constraintpresence, colorlimit, colorlimit-con-straintpresence, priceupperlimit, priceupperlimit-constraintpresence • agreement state: influence-on-listener, commit-speaker, solution-size, prev-influence-on-lis- tener, prev-commit-speaker, prev-solution-size, distance-of-last-state-in-utterances, distance-of-last-state-in-turns, ref-made-in-prev-action-state, speaker-of-last-state, prev-ref-state • previous agreement state description: prev-state-type-expressed, prev-state-color-expressed, prev-state-owner-expressed, prev-state-price-expressed, prev-state-quantity-expressed • solution interactions: color-contrast, price-contrast Figure 10: Intentional Influences Feature Set. The task situation features encode inferable changes in the task situation that are related to item attributes, where colormatch is a boolean feature that indicates whether there hasbeen a change in the color match constraint. The pricelimit, colorlimit and priceupperlimitfeatures are also boolean features representing that there has been a constraint changerelated to setting limits on values for the price and color attributes. The features withconstraintpresence appended to a constraint feature name are symbolic features that indicatewhether the constraint change was implicit or explicit. For example, if there is an agreedupon constraint to try to select items with the same color value for a room, and a speakerwants to relax that constraint then the feature colormatch would have the value yes. If thespeaker communicated this explicitly by saying “Let’s forget trying to match colors.” thenthe constraintpresence feature would have the value explicit and otherwise it would havethe value implicit. If the constraint change is not explicitly communicated and the speakerdecides to include a color attribute when it is not necessary for identification purposes, itmay be to help the hearer infer that he means to drop the constraint The agreement state features in Figure 10 encode critical points of agreement during problem solving. Critical agreement states are (Di Eugenio et al., 2000): • propose: the speaker offers the entity and this conditional commitment results in a determinate solution size. • partner decidable option: the speaker offers the entity and this conditional commit- ment results in an indeterminate solution size. • unconditional commit: the speaker commits to an entity. • unendorsed option: the speaker offers the entity but does not show any commitment to using it when the solution size is already determinate. For example, if a dialogue participant is unconditionally committing in response to a proposal, she may want to verify that she has the same item and the same entity de-scription as her partner by repeating back the previous description. The features thatencode these critical agreement states include some DAMSL features (influence-on-listener, 173

Jordan & Walker commit-speaker, prev-influence-on-listener, prev-commit-speaker), progress toward a solu-tion (solution-size, prev-solution-size, ref-made-in-prev-action-state), and features inherentto an agreement state (speaker-of-last-state, distance-of-last-state-in-utterances, distance-of-last-state-in-turns). The features that make reference to a state are derived from theagreement state features and a more extensive discourse history than can be encoded withinthe feature representation. In addition, since the current agreement state depends in parton the previous agreement state, we added the derived agreement state. The previousagreement state description features in Figure 10 are booleans that capture dependenciesof the model on the content of the description from a previous state. For example, if theprevious agreement state for an entity expressed only type and color attributes then thiswould be encoded yes for prev-state-type-expressed and prev-state-color-expressed and nofor the rest. The solution interactions features in Figure 10 represent situations where multiple pro- posals are under consideration which may contrast with one another in terms of solvingcolor-matching goals (color-contrast) or price related goals (price-contrast). When the boolean feature color-contrast is true, it means that the entity’s color matches with thepartial solution that has already been agreed upon and contrasts with the alternatives thathave been proposed. In this situation, there may be grounds for endorsing this entity rel-ative to the alternatives. For example, in response to S’s utterance [37] in Figure 1, in acontext where G earlier introduced one blue rug for 175, G could have said “Let’s use myblue rug.” in response. In this case the blue rug would have a true value for color-contrastbecause it has a different color than the alternative, and it matches the blue sofa that hadalready been selected. The boolean feature price-contrast describes two different situations. When the feature price-contrast is true, it either means that the entity has the best price relative to thealternatives, or when the problem is nearly complete, that the entity is more expensivethan the alternatives. In the first case, the grounds for endorsement are that the item ischeaper. In the second case, it may be that the item will spend out the remaining budgetwhich will result in a higher score for the problem solution. Note that although the solution interaction features depend upon the agreement states, in that it is necessary to recognize proposals and commitments in order to identify alter-natives and track agreed upon solutions, it is difficult to encode such extensive historicalinformation directly in a feature representation. Therefore the solution interaction featuresare derived, and the derivation includes heuristics that use agreement state features forestimating partial solutions. A sample encoding for the dialogue excerpt in Figure 1 for itsproblem solving utterance level annotations and agreement states were given in Figures 3and 4. 3.3 Brennan and Clark’s Conceptual Pact Model Brennan and Clark’s conceptual pact model focuses on the bidirectional adaptation ofeach conversational partner to the linguistic choices of the other conversational participant.The conceptual pact model suggests that dialogue participants negotiate a descriptionthat both find adequate for describing an object (Clark & Wilkes-Gibbs, 1986; Brennan& Clark, 1996). The speaker generates trial descriptions that the hearer modifies based 174

Learning Content Selection Rules for Generating Object Descriptions on which object he thinks he is suppose to identify. The negotiation continues until theparticipants are confident that the hearer has correctly identified the intended object. Brennan and Clark (1996) further point out that lexical availability, perceptual salience and a tendency for people to reuse the same terms when describing the same object in aconversation, all significantly shape the descriptions that people generate. These factorsmay then override the informativeness constraints imposed by Grice’s Quantity Maxim.Lexical availability depends on how an object is best conceptualized and the label associatedwith that conceptualization (e.g. is the referent “an item of furniture” or “a sofa”). Withperceptual salience, speakers may include a highly salient attribute rather than just theattributes that distinguish it from its distractors, e.g. “the 50 red sofa” when “the 50sofa” may be informative enough. Adaptation to one’s conversational partner should leadto a tendency to reuse a previous description. The tendency to reuse a description derives from a combination of the most recent, successfully understood description of the object, and how often the description has beenused in a particular conversation. However, this tendency is moderated by the need toadapt a description to changing problem-solving circumstances and to make those repeateddescriptions even more efficient as their precedents become more established for a particularpairing of conversational partners. Recency and frequency effects on reuse are reflectionsof a coordination process between conversational partners in which they are negotiating ashared way of labelling or conceptualizing the referent. Different descriptions may be trieduntil the participants agree on a conceptualization. A change in the problem situation maycause the conceptualization to be embellished with additional attributes or may instigatethe negotiation of a new conceptualization for the same referent. The additional features suggested by this model include the previous description since that is a candidate conceptual pact, how long ago the description was made, and howfrequently it was referenced. If the description was used further back in the dialogue or wasreferenced frequently, that could indicate that the negotiation process had been completed.Furthermore, the model suggests that, once a pact has been reached, that the dialogueparticipants will continue to use the description that they previously negotiated unless theproblem situation changes. The continued usage aspect of the model is also similar toPassonneau’s lexical focus model (Passonneau, 1995). • interactions with other discourse entities: distance-last-ref, distance-last-ref-in-turns, number- prev-mentions, speaker-of-last-ref, distance-last-related • previous description: color-in-last-exp, type-in-last-exp, owner-in-last-exp, price-in-last-exp, quantity-in-last-exp, type-in-last-turn, color-in-last-turn, owner-in-last-turn, price-in-last-turn, quantity-in-last-turn, initial-in-last-turn • frequency of attributes: freq-type-expressed, freq-color-expressed, freq-price-expressed, freq- owner-expressed, freq-quantity-expressed • stability history: cp-given-last-2, cp-given-last-3 Figure 11: conceptual pact Feature Set. 175

Jordan & Walker The conceptual pact features in Figure 11 encode how the current description relates to previous descriptions of the same entity. We encode recency information: when the entitywas last described in terms of number of utterances and turns (distance-last-ref, distance-last-in-turns), when the last related description (e.g. set, class) was (distance-last-related),how frequently it was described (number-prev-mentions), who last described it (speaker-of-last-ref), and how it was last described in terms of turn and expression since the descriptionmay have been broken into several utterances (color-in-last-exp, type-in-last-exp, owner-in-last-exp, price-in-last-exp, quantity-in-last-exp, type-in-last-turn, color-in-last-turn, owner-in-last-turn, price-in-last-turn, quantity-in-last-turn, initial-in-last-turn). We also encodefrequency information: the frequency with which attributes were expressed in previousdescriptions of it (freq-type-expressed, freq-color-expressed, freq-price-expressed, freq-owner-expressed, freq-quantity-expressed), and a history of possible conceptual pacts that mayhave been formed; the attribute types used to describe it in the last two and last threedescriptions of it if they were consistent across usages (cp-given-last-2, cp-given-last-3). 4. Experimental Method The experiments utilize the rule learning program ripper (Cohen, 1996) to learn the contentselection component of an object description generator from the object descriptions in the coconut corpus. Although any categorization algorithm could be applied to this problemgiven the current formulation, ripper is a good match for this particular setup becausethe if-then rules that are used to express the learned model can be easily compared withthe theoretical models of content selection described above. One drawback is that ripperdoes not automatically take context into account during training so the discourse contextmust be represented via features as well. Although it might seem desirable to use ripper’sown previous predictions as additional context during training, since it will consider themin practice, it is unnecessary and irrelevant to do so. The learned model will consist ofgeneration rules that are relative to what is in the discourse as encoded features (i.e. whatwas actually said in the corpus) and any corrections it learns are only good for improvingperformance on a static corpus. Like other learning programs, ripper takes as input the names of a set of classes to be learned, the names and ranges of values of a fixed set of features, and training data specifyingthe class and feature values for each example in a training set. Its output is a classificationmodel for predicting the class of future examples. In ripper, the classification model islearned using greedy search guided by an information gain metric, and is expressed as anordered set of if-then rules. By default ripper corrects for noisy data. In the experimentsreported here, unlike those reported by Jordan and Walker (2000), corrections for noisydata have been suppressed since the reliability of the annotated features is high. Thus to apply ripper, the object descriptions in the corpus are encoded in terms of a set of classes (the output classification), and a set of input features that are used as predictorsfor the classes. As mentioned above, the goal is to learn which of a set of content attributesshould be included in an object description. Below we describe how a class is assigned toeach object description, summarize the features extracted from the dialogue in which eachexpression occurs, and the method applied to learn to predict the class of object descriptionfrom the features. 176

Learning Content Selection Rules for Generating Object Descriptions N in Explicit attributes in Class Name Corpus object description CPQ 64 Color, Price, Quantity CPO 56 Color, Price, Owner CPOQ 46 Color, Price, Owner, Quantity T 42 None (type only) CP 41 Color, Price O 32 Owner CO 31 Color, Owner C 18 Color CQ 14 Color, Quantity COQ 13 Color, Owner, Quantity OQ 12 Owner, Quantity PO 11 Price, Owner Q 5 Quantity P 4 Price PQ 2 Price, Quantity POQ 2 Price, Owner, Quantity Figure 12: Encoding of attributes to be included in terms of ML Classes, ordered by fre- quency 4.1 Class Assignment The corpus of object descriptions is used to construct the machine learning classes as follows. The learning task is to determine the subset of the four attributes, color, price,owner, quantity, to include in an object description. Thus one method for representingthe class that each object description belongs to is to encode each object description asa member of the category represented by the set of attributes expressed by the objectdescription. This results in 16 classes representing the power set of the four attributes asshown in Figure 12. The frequency of each class is also shown in Figure 12. Note that theseclasses are encodings of the hand annotated explicit attributes that were shown in Figure 6but exclude the type attribute since we are not attempting to model pronominal selections. 4.2 Feature Extraction The corpus is used to construct the machine learning features as follows. In ripper, featurevalues are continuous (numeric), set-valued, or symbolic. We encoded each discourse entityfor a furniture item in terms of the set of 82 total features described in Section 3 as inspiredby theories of content selection for subsequent reference. These features were either directlyannotated by humans as described in Section 2, derived from annotated features, or inherentto the dialogue (Di Eugenio et al., 2000; Jordan, 2000b). The dialogue context in whicheach description occurs is directly represented in the encodings. In a dialogue system, thedialogue manager would have access to all these features, which are needed by the problemsolving component, and would provide them to the language generator. The entire featureset is summarized in Figure 13. 177

Jordan & Walker • Assumed Familiarity Features – mutually known attributes: type-mk, color-mk, owner-mk, price-mk, quantity-mk – reference-relation: one of initial, coref, set, class, cnanaphora, predicative • Inherent Features – utterance-number, speaker-pair, speaker, problem-number – attribute values: ∗ type: one of sofa, chair, table, rug, lamp, superordinate∗ color: one of red, blue, green, yellow∗ owner: one of self, other, ours∗ price: range from 50 to 600 ∗ quantity: range from 0 to 4. • Conceptual Pact Features – interactions with other discourse entities: distance-last-ref, distance-last-ref-in-turns, number-prev-men- tions, speaker-of-last-ref, distance-last-related – previous description: color-in-last-exp, type-in-last-exp, owner-in-last-exp, price-in-last-exp, quantity- in-last-exp, type-in-last-turn, color-in-last-turn, owner-in-last-turn, price-in-last-turn, quantity-in-last-turn, initial-in-last-turn – frequency of attributes: freq-type-expressed, freq-color-expressed, freq-price-expressed, freq-owner- expressed, freq-quantity-expressed – stability history: cp-given-last-2, cp-given-last-3 • Contrast Set Features – distractor frequencies: type-distractors, color-distractors, owner-distractors, price-distractors, quantity- distractors – Attribute Saliency: majority-type, majority-type-freq, majority-color, majority-color-freq, majority- price, majority-price-freq, majority-owner, majority-owner-freq, majority-quantity, majority-quantity-freq • Intentional Influences Features – task situation: goal, colormatch, colormatch-constraintpresence, pricelimit, pricelimit-constraintpres- ence, priceevaluator, priceevaluator-constraintpresence, colorlimit, colorlimit-constraintpresence, price-upperlimit, priceupperlimit-constraintpresence – agreement state: influence-on-listener, commit-speaker, solution-size, prev-influence-on-listener, prev- commit-speaker, prev-solution-size, distance-of-last-state-in-utterances, distance-of-last-state-in-turns,ref-made-in-prev-action-state, speaker-of-last-state, prev-ref-state – previous agreement state description: prev-state-type-expressed, prev-state-color-expressed, prev-state- owner-expressed, prev-state-price-expressed, prev-state-quantity-expressed – solution interactions: color-contrast, price-contrast Figure 13: Full Feature Set for Representing Basis for Object Description Content Selection in Task Oriented Dialogues. 4.3 Learning Experiments The final input for learning is training data, i.e., a representation of a set of discourseentities, their discourse context and their object descriptions in terms of feature and class 178

Learning Content Selection Rules for Generating Object Descriptions values. In order to induce rules from a variety of feature representations, the training datais represented differently in different experiments. The goal of these experiments is to test the contribution of the features suggested by the three models of object description content selection described in Section 3. Our predictionis that the incremental and the intentional influences models will work best incombination for predicting object descriptions for both initial and subsequent reference.This is because: (1) the intentional influences features capture nothing relevant tothe reference identification goal, which is the focus of the incremental model, and (2) wehypothesize that the problem solving state will be relevant for selecting attributes for initialdescriptions, and the incremental model features capture nothing directly about theproblem solving state, but this is the focus of the intentional influences model. Finallywe expect the conceptual pact model to work best in conjunction with the incrementaland the intentional influences models since it is overriding informativeness constraints,and since, after establishing a pact, it may need to adapt the description to make it moreefficient or re-negotiate the pact as the problem-solving situation changes. Therefore, examples are first represented using only the assumed familiarity features in Figure 7 to establish a performance baseline for assumed familiarity information. Wethen add individual feature sets to the assumed familiarity feature set to examine thecontribution of each feature set on its own. Thus, examples are represented using only thefeatures specific to a particular model, i.e. the conceptual pact features in Figure 11, thecontrast set features in Figure 9 or the intentional influences features in Figure 10.Remember that there are three different versions of the contrast set features, derivedfrom three different models of what is currently “in focus”. One model (segment) is basedon intentional structure (Grosz & Sidner, 1986). The other two are simple recency basedmodels where the active focus space either contains only discourse entities from the mostrecent utterance or the most recent five utterances (one utterance, five utterance). In addition to the theoretically-inspired feature sets, we include the task and dialogue specific inherent features in Figure 8. These particular features are unlikely to producerules that generalize to other domains, including new coconut dialogues, because eachdomain and pair of speakers will instantiate these values uniquely for a particular domain.Thus, these features may indicate aspects of individual differences, and the role of thespecific situation in models of content selection for object descriptions. Next, examples are represented using combinations of the features from the different models to examine interactions between feature sets. Finally, to determine whether particular feature types have a large impact (e.g. fre- quency features), we report results from a set of experiments using singleton feature sets,where those features that varied by attribute alone are clustered into sets while the restcontain just one feature. For example, the distractor frequency attributes in Figure 9 forma cluster for a singleton feature set whereas utterance-number is the only member of itsfeature set. We experimented with singleton feature sets in order to determine if any aremaking a large impact on the performance of the model feature set to which they belong. The output of each machine learning experiment is a model for object description gen- eration for this domain and task, learned from the training data. To evaluate these models,the error rates of the learned models are estimated using 25-fold cross-validation, i.e. the to-tal set of examples is randomly divided into 25 disjoint test sets, and 25 runs of the learning 179

Jordan & Walker program are performed. Thus, each run uses the examples not in the test set for trainingand the remaining examples for testing. An estimated error rate is obtained by averagingthe error rate on the test portion of the data from each of the 25 runs. For sample sizes inthe hundreds (the coconut corpus provides 393 examples), cross-validation often providesa better performance estimate than holding out a single test set (Weiss & Kulikowski, 1991).The major advantage is that in cross-validation all examples are eventually used for testing,and almost all examples are used in any given training run. 5. Experimental Results Table 2 summarizes the experimental results. For each feature set, and combination offeature sets, we report accuracy rates and standard errors resulting from 25-fold cross-validation. We test differences in the resulting accuracies using paired t-tests. The tableis divided into regions grouping results using similar feature sets. Row 1 provides the accuracy for the majority class baseline of 16.9%; this is the standard baseline thatcorresponds to the accuracy achieved from simply choosing the description type that occursmost frequently in the corpus, which in this case means that the object description generatorwould always use the color, price and quantity attributes to describe a domain entity. Row2 provides a second baseline, namely that for using the assumed familiarity featureset. This result shows that providing the learner with information about whether the values of the attributes for a discourse entity are mutually known does significantly improveperformance over the majority class baseline (t=2.4, p< .03). Examination of the restof the table shows clearly that the accuracy of the learned object description generatordepends on the features that the learner has available. Rows 3 to 8 provide the accuracies of object description generators trained and tested using one of the additional feature sets in addition to the familiarity feature set. Overall,the results here show that compared to the familiarity baseline, the features for inten-tional influences (familiarity,iinf t=10.0, p<.01), contrast set (familiarity,segt=6.1, p< .01; familiarity,1utt t=4.7, p< .01; familiarity,5utt t=4.2, p< .01), andconceptual pact (familiarity,cp t=6.2, p< .01) taken independently significantly im-prove performance. The accuracies for the intentional influences features (Row 7) aresignificantly better than for conceptual pact (t=5.2, p<.01) and the three parameteriza-tions of the incremental model (familiarity,seg t=6, p<.01; familiarity,1utt t=4.3,p<.01; familiarity,5utt t=4.2, p<.01), perhaps indicating the importance of a directrepresentation of the problem solving state for this task. In addition, interestingly, Rows 3, 4 and 5 show that features for the incremental model that are based on the three different models of discourse structure all perform equallywell, i.e. there are no statistically significant differences between the distractors predicted bythe model of discourse structure based on intention (seg) and the two recency based models(1utt, 5utt), even though the raw accuracies for distractors predicted by the intention-based model are typically higher.6 The remainder of the table shows that the intentionbased model only performs better than a recency based model when it is combined with allfeatures (Row 15 seg vs. Row 16 1utt t=2.1, p<.05). 6. This is consistent with the findings reported by Jordan (2000b) which used a smaller dataset to measure which discourse structure model best explained the data for the incremental model. 180

Learning Content Selection Rules for Generating Object Descriptions Row Model Tested Feature Sets Used Accuracy (SE) 1 baseline majority class 16.9% (2.1) 2 baseline familiarity 18.1% (2.1) 3 incremental familiarity,seg 29.0% (2.2) 4 incremental familiarity,1utt 29.0% (2.5) 5 incremental familiarity,5utt 30.4% (2.6) 6 conceptual pact familiarity,cp 28.9% (2.1) 7 intentional influences familiarity,iinf 42.4% (2.7) 8 situation specific familiarity,inh 54.5% (2.3) 9 intentional influences, incremental familiarity,iinf,seg 46.6% (2.2) 10 intentional influences, incremental familiarity,iinf,1utt 42.7% (2.2) 11 intentional influences, incremental familiarity,iinf,5utt 44.4% (2.6) 12 all theory features combined familiarity,iinf,cp,seg 43.2% (2.8) 13 all theory features combined familiarity,iinf,cp,1utt 40.9% (2.6) 14 all theory features combined familiarity,iinf,cp,5utt 41.9% (3.2) 15 all theories & situation specific familiarity,iinf,inh,cp,seg 59.9% (2.4) 16 all theories & situation specific familiarity,iinf,inh,cp,1utt 55.4% (2.2) 17 all theories & situation specific familiarity,iinf,inh,cp,5utt 57.6% (3.0) 18 best singletons familiarity,iinf,inh,cp,seg 52.9% (2.9) from all models combined 19 best singletons familiarity,iinf,inh,cp,1utt 47.8% (2.4) from all models combined 20 best singletons familiarity,iinf,inh,cp,5utt 50.3% (2.8) from all models combined Table 2: Accuracy rates for the content selection component of a object description genera- tor using different feature sets, SE = Standard Error. cp = the conceptual pactfeatures. iinf = the intentional influences features. inh = the inherent fea-tures. seg = the contrast-set, segment focus space features. 1utt = thecontrast set, one utterance focus space features, 5utt = the contrastset, five utterance focus space features. Finally, the situation specific model based on the inherent feature set (Row 8) which is domain, speaker and task specific performs significantly better than the famil-iarity baseline (t=16.6, p< .01). It is also significantly better than any of the modelsutilizing theoretically motivated features. It is significantly better than the intentionalinfluences model (t=5, p<.01), and the conceptual pact model (t=9.9, p<.01), aswell as the three parameterizations of the incremental model (seg t=10, p<.01; 1uttt=10.4, p<.01; 5utt t=8.8, p<.01). 181

Jordan & Walker Say POQ if priceupperlimit-constraintpresence = IMPLICIT ∧ reference-relation = classSay COQ if goal = SELECTCHAIRS ∧ colormatch-constraintpresence = IMPLICIT ∧ prev-solution-size =DETERMINATE ∧ reference-relation = corefSay COQ if goal = SELECTCHAIRS ∧ distance-of-last-state-in-utterances >= 3 ∧ speaker-of-last-state = SELF ∧reference-relation = initialSay COQ if goal = SELECTCHAIRS ∧ prev-ref-state = STATEMENT ∧ influence-on-listener = action-directive ∧prev-solution-size = DETERMINATESay C if prev-commit-speaker = commit ∧ influence-on-listener = action-directive ∧ color-contrast = no ∧speaker-of-last-state = SELFSay C if color-contrast = yes ∧ goal = SELECTTABLE ∧ prev-influence-on-listener = action-directive ∧ influence-on-listener = naSay C if solution-size = DETERMINATE ∧ prev-influence-on-listener = na ∧ prev-state-color-expressed = yes ∧prev-state-price-expressed = na ∧ prev-solution-size = DETERMINATESay CO if colorlimit = yesSay CO if price-mk = yes ∧ prev-solution-size = INDETERMINATE ∧ price-contrast = yes ∧ commit-speaker = naSay CO if price-mk = yes ∧ prev-ref-state = PARTNER-DECIDABLE-OPTION ∧ distance-of-last-state-in-utterances <= 1 ∧ prev-state-type-expressed = yesSay O if prev-influence-on-listener = open-option ∧ reference-relation = corefSay O if influence-on-listener = info-request ∧ distance-of-last-state-in-turns <= 0Say CP if solution-size = INDETERMINATE ∧ price-contrast = yes ∧ distance-of-last-state-in-turns >= 2Say CP if distance-of-last-state-in-utterances <= 1 ∧ goal = SELECTSOFA ∧ influence-on-listener = na ∧reference-relation = classSay T if prev-solution-size = DETERMINATE ∧ distance-of-last-state-in-turns <= 0 ∧ prev-state-type-expressed= yes ∧ ref-made-in-prev-action-state = yesSay T if prev-solution-size = DETERMINATE ∧ colormatch-constraintpresence = EXPLICITSay T if prev-solution-size = DETERMINATE ∧ goal = SELECTSOFA ∧ prev-state-owner-expressed = na ∧color-contrast = noSay CPOQ if goal = SELECTCHAIRS ∧ prev-solution-size = INDETERMINATE ∧ price-contrast = no ∧ type-mk= noSay CPOQ if distance-of-last-state-in-utterances >= 5 ∧ type-mk = noSay CPOQ if goal = SELECTCHAIRS ∧ influence-on-listener = action-directive ∧ distance-of-last-state-in-utterances >= 2Say CPO if influence-on-listener = action-directive ∧ distance-of-last-state-in-utterances >= 2 ∧ commit-speaker =offerSay CPO if goal = SELECTSOFA ∧ distance-of-last-state-in-utterances >= 1default Say CPQ Figure 14: A Sampling of Rules Learned Using assumed familiarity and intentional influences Features. The classes encode the four attributes, e.g CPOQ = Color,Price,Owner and Quantity, T = Type only In Section 4.3, we hypothesized that the incremental and intentional influences models would work best in combination. Rows 9, 10 and 11 show the results of this com-bination for each underlying model of discourse structure. Each of these combinationsprovides some increase in accuracy, however the improvements in accuracy over the objectdescription generator based on the intentional influences features alone (Row 7) arenot statistically significant. Figure 14 shows the rules that the object description generator learns given the as- sumed familiarity and intentional influences features. The rules make use of bothtypes of assumed familiarity features and all four types of intentional influencesfeatures. The features representing mutually known attributes and those representing theattributes expressed in a previous agreement state can be thought of as overlapping with 182

Learning Content Selection Rules for Generating Object Descriptions the conceptual pact model, while features representing problem-solving structure andagreement state may overlap with the incremental model by indicating what is in focus. One of the rules from the rule set in Figure 14 is: Say T if prev-solution-size = DETERMINATE ∧ colormatch-constraintpresence = EXPLICIT . An example of a dialogue excerpt that matches this rule is shown in Figure 15. The rule captures a particular style of problem solving in the dialogue in which the conversantstalk explicitly about the points involved in matching colors (we only get 650 points withoutrug and bluematch in living room) to argue for including a particular item (rug). In thiscase, because a solution had been proposed, the feature prev-solution-size has the valuedeterminate. So the rule describes those contexts in which a solution has been counter-proposed, and support for the counter-proposal is to be presented. D: I suggest that we buy my blue sofa 300, your 1 table high green 200, your 2 chairs red 50, my 2chairs red 50 and you can decide the rest. What do you thinkJ: your 3 chair green my high table green 200 and my 1 chair green 100. your sofa blue 300 rug blue250. we get 700 point. 200 for sofa in livingroom plus rug 10. 20 points for match. 50 points formatch in dining room plus 20 for spending all. red chairs plus red table costs 600. we only get 650points without rug and bluematch in living room. add it up and tell me what you think. Figure 15: Example of a discourse excerpt that matches a rule in the intentional influ- ences and assumed familiarity rule set Rows 12, 13 and 14 in Table 2 contain the results of combining the conceptual pact features with the intentional influences features and the contrast set features.These results can be directly compared with those in Rows 9, 10 and 11. Because rip-per uses a heuristic search, the additional features have the effect of making the accuraciesfor the resulting models lower. However, none of these differences are statistically signifi-cant. Taken together, the results in Rows 9-14 indicate that the best accuracies obtainablewithout using situation specific features (the inherent feature set), is the combination ofthe intentional influences and contrast set features, with a best overall accuracyof 46.6% as shown in Row 9. Rows 15, 16 and 17 contain the results for combining all the features, including the inherent feature set, for each underlying model of discourse structure. This time there isone significant difference between the underlying discourse models in which the intention-based model, segment, is significantly better than the one utterance recency model(t=2.1, p<.05) but not the five utterance recency model. Of the models in this grouponly the segment model is significantly better than the models that use a subset of thefeatures (vs. inherent t=2.4, p<.03). Figure 16 shows the generation rules learned withthe best performing feature set shown in Row 15. Many task, entity and speaker specificfeatures from the inherent feature set are used in these rules. This rule set performsat 59.9% accuracy, as opposed to 46.6% accuracy for the more general feature set (shownin Row 9). In this final rule set, no conceptual pact features are used and removing 183

Jordan & Walker Say Q if type=CHAIR ∧ price>=200 ∧ reference-relation=set ∧ quantity>=2.Say Q if speaker=GARRETT ∧ color-distractors<=0 ∧ type=CHAIR.Say PO if color=unk ∧ speaker-pair=GARRETT-STEVE ∧ reference-relation=initial ∧ color-contrast=no.Say PO if majority-color-freq>=6 ∧ reference-relation=set.Say PO if utterance-number>=39 ∧ type-distractors<=0 ∧ owner=SELF ∧ price>=100.Say OQ if color=unk ∧ quantity>=2 ∧ majority-price-freq<=5.Say OQ if prev-state-quantity-expressed=yes ∧ speaker=JULIE ∧ color=RED.Say COQ if goal=SELECTCHAIRS ∧ price-distractors<=3 ∧ owner=SELF ∧ distance-of-last-state-in- utterances>=3 ∧ majority-price<=200.Say COQ if quantity>=2 ∧ price<=-1 ∧ ref-made-in-prev-action-state=no.Say COQ if quantity>=2 ∧ price-distractors<=3 ∧ quantity-distractors>=4 ∧ influence-on-listener=action-directive.Say CQ if speaker-pair=DAVE-GREG ∧ utterance-number>=22 ∧ utterance-number<=27 ∧ problem<=1.Say CQ if problem>=2 ∧ quantity>=2 ∧ price<=-1.Say CQ if color=YELLOW ∧ quantity>=3 ∧ influence-on-listener=action-directive ∧ type=CHAIR.Say C if price-mk=yes ∧ majority-type=SUPERORDINATE ∧ quantity-distractors>=3.Say C if price-mk=yes ∧ utterance-number<=21 ∧ utterance-number>=18 ∧ prev-state-price-expressed=na ∧majority-price>=200 ∧ color-distractors>=2.Say CO if utterance-number>=16 ∧ price<=-1 ∧ type=CHAIR.Say CO if price-mk=yes ∧ speaker-pair=JILL-PENNY.Say CO if majority-price<=75 ∧ distance-of-last-state-in-utterances>=4 ∧ prev-state-type-expressed=na.Say O if color=unk ∧ speaker-pair=GARRETT-STEVE.Say O if color=unk ∧ owner=OTHER ∧ price<=300.Say O if prev-influence-on-listener=open-option ∧ utterance-number>=22.Say CP if problem>=2 ∧ quantity<=1 ∧ type=CHAIR.Say CP if price>=325 ∧ reference-relation=class ∧ distance-of-last-state-in-utterances<=0.Say CP if speaker-pair=JON-JULIE ∧ type-distractors<=1.Say CP if reference-relation=set ∧ owner=OTHER ∧ owner-distractors<=0.Say T if prev-solution-size=DETERMINATE ∧ price>=250 ∧ color-distractors<=5 ∧ owner-distractors>=2 ∧utterance-number>=15.Say T if color=unk.Say T if prev-state-type-expressed=yes ∧ distance-of-last-state-in-turns<=0 ∧ owner-distractors<=4.Say CPOQ if goal=SELECTCHAIRS ∧ prev-solution-size=INDETERMINATE.Say CPOQ if speaker-pair=KATHY-MARK ∧ prev-solution-size=INDETERMINATE ∧ owner-distractors<=5.Say CPOQ if goal=SELECTCHAIRS ∧ influence-on-listener=action-directive ∧ utterance-number<=22.Say CPO if utterance-number>=11 ∧ quantity<=1 ∧ owner-distractors>=1.Say CPO if influence-on-listener=action-directive ∧ price>=150.Say CPO if reference-relation=class ∧ problem<=1.default Say CPQ Figure 16: A Sampling of the Best Performing Rule Set. Learned using the assumed fa- miliarity, inherent, intentional influences and contrast set featuresets. The classes encode the four attributes, e.g., CPOQ = Color,Price,Ownerand Quantity, T = Type only. these features during training had no effect on accuracy. All of the types of features inthe assumed familiarity, inherent, and contrast set are used. Of the intentionalinfluences features, mainly the agreement state and previous agreement state descriptionsare used. Some possible explanations are that the agreement state is a stronger influencethan the task situation or that the task situation is not modelled well. Why does the use of the inherent feature set contribute so much to overall accuracy and why are so many inherent features used in the rule set in Figure 16? It may be thatthe inherent features of objects would be important in any domain because there is a lotof domain specific reasoning in the task of object description content selection. However,these features are most likely to support rules that overfit to the current data set; as we 184

Learning Content Selection Rules for Generating Object Descriptions have said before, rules based on the inherent feature set are unlikely to generalize to newsituations. However, there might be more general or abstract versions of these featuresthat could generalize to new situations. For example, the attribute values for the discourseentity may be capturing aspects of the problem solving (e.g. near the end of the problem,the price of expensive items is highly relevant). Second, the use of utterance-numbers canbe characterized as rules about the beginning, middle and end of a dialogue and may againreflect problem solving progress. Third, the rules involving problem-number suggest thatthe behavior for the first problem is different from the others and may reflect that thedialogue partners have reached an agreement on their problem solving strategy. Finally,the use of speaker-pair features in the rules included all but two of the possible speaker-pairs, which may reflect differences in the agreements reached on how to collaborate. Oneof the rules from this rule set is shown below: Say CP if price >= 325 ∧ reference-relation = class ∧ distance-of-last-state- in-utterances <= 0. This rule applies to discourse entities in the dialogues of one speaker pair. An example dialogue excerpt that matches this rule is in Figure 17. The rule reflects a particular styleof describing the items that are available to use in the problem solving, in which the speakerfirst describes the class of the items that are about to be listed. This style of descriptionallows the speaker to efficiently list what he has available. The distance-of-last-state-in-utterances feature captures that this style of description occurs before any proposals havebeen made. M: I have 550, my inventory consists of 2 Yellow hi-tables for 325 each. Sofas, yellow for 400and green for $350. Figure 17: Example of a dialogue excerpt that matches a rule in the best performing rule set As described above, we also created singleton feature sets, in addition to our theoreti- cally inspired feature sets, to determine if any singleton features are, by themselves, makinga large impact on the performance of the model it belongs to. The singleton features shownin Table 3 resulted in learned models that were significantly above the majority class base-line. The last column of Table 3 also shows that, except for the assumed familiarity andincremental 5utt models, the theory model to which a particular singleton feature be-longs is significantly better, indicating that no singleton alone is a better predictor than thecombined features in these theoretical models. The assumed familiarity and incremen-tal 5utt models perform similarly to their corresponding single feature models indicatingthat these single features are the most highly useful features for these two models. Finally, we combined all of the singleton features in Table 3 to learn three additional models shown in Rows 18, 19 and 20 of Table 2. These three models are not significantlydifferent from one another. The best performing model in Row 15, which combines all 185

Jordan & Walker Source Features in Set Accuracy Better than Source Model Model (SE) baseline Better assumed fa- type-mk, color-mk, owner-mk, 18.1% (2.1) t=2.4, p<.03 identical miliarity price-mk, quantity-mk conceptual freq-type-expressed, freq-color- 22.1% (1.8) t=3.7, p<.01 t=5.7, p<.01 pact expressed, freq-price-expressed,freq-owner-expressed, freq- quantity-expressedcp-given-last-2 20.9% (2.1) t=3.9, p<.01 t=4.3, p<.01 type-in-last-exp, color-in-last- 18.9% (1.9) t=2.8, p<.02 t=5.7, p<.01 exp, price-in-last-exp, owner-in-last-exp, quantity-in-last-exptype-in-last-turn, color-in- 18.1% (2.0) t=3.4, p<.02 t=6.4, p<.01 last-turn, price-in-last-turn, owner-in-last-turn, quantity-in-last-turn incremental type-distractors, color- 21.4% (2.5) t=3.2, p<.01 t=3.6, p<.01 seg distractors, price-distractors, owner-distractors, quantity- distractorsmajority-type, majority-color, 19.9% (2.3) t=2.5, p<.02 t=4.8, p<.01 majority-price, majority-owner,majority-quantity incremental type-distractors, color- 20.8% (2.4) t=3.2, p<.01 t=3.2, p<.01 1utt distractors, price-distractors, owner-distractors, quantity- distractors incremental type-distractors, color- 25.7% (2.7) t=4.4, p<.01 t=1.5, NS 5utt distractors, price-distractors, owner-distractors, quantity- distractors intentional distance-of-last-state-in- 21.3% (2.0) t=3.7, p<.01 t=11, p<.01 influences utterancesdistance-of-last-state-in-turns 20.0% (2.1) t=3.6, p<.01 t=10.2, p<.01 colormatch 19.3% (2.2) t=3.7, p<.01 t=10.3, p<.01 prev-state-type-expressed, 19.2% (1.9) t=3.6, p<.01 t=8.8, p<.01 prev-state-color-expressed,prev-state-owner-expressed,prev-state-price-expressed,prev-state-quantity-expressed situation type, color, price, owner, quan- 24.3% (2.5) t=4.1, p<.01 t=12.5, p<.01 specific tityutterance-number 20.5% (2.3) t=3.3, p<.01 t=16.2, p<.01 Table 3: Performance using singleton feature sets, SE = Standard Error features, is significantly better than 1utt (t=4.2, p<.01) and 5utt (t=2.8, p<.01) in Rows19 and 20, but is not significantly different from seg (t=2.0, NS) in Row 18. The combinedsingletons seg model (Row 18) is also not significantly different from the inherent model 186

Learning Content Selection Rules for Generating Object Descriptions (Row 8). The combined singletons seg model has the advantage that it requires just twosituation specific features and a smaller set of theoretical features. Class recall precision fallout F (1.00) CPQ 100.00 63.64 12.12 0.78 CPO 66.67 100.00 0.00 0.80 CPOQ 100.00 100.00 0.00 1.00 T 50.00 100.00 0.00 0.67 CP 100.00 100.00 0.00 1.00 O 100.00 60.00 5.41 0.75 CO 66.67 100.00 0.00 0.80 C 0.00 0.00 5.13 0.00 CQ 0.00 100.00 0.00 0.00 COQ 100.00 100.00 0.00 1.00 PO 50.00 100.00 0.00 0.67 OQ 66.67 50.00 5.41 0.57 Q 0.00 0.00 2.50 0.00 POQ 0.00 100.00 0.00 0.00 PQ 0.00 100.00 0.00 0.00 Table 4: Recall and Precision values for each class; the rows are ordered from most frequent to least frequent class Class CPQ O COQ C CPO CO PO T OQ POQ CPOQ Q CP PQ CQ CPQ 7 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 COQ 0 0 2 0 0 0 0 0 0 0 0 0 0 0 0 C 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CPO 0 1 0 1 4 0 0 0 0 0 0 0 0 0 0 CO 1 0 0 0 0 2 0 0 0 0 0 0 0 0 0 PO 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 T 0 0 0 1 0 0 0 1 0 0 0 0 0 0 0 OQ 0 0 0 0 0 0 0 0 2 0 0 1 0 0 0 POQ 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 CPOQ 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0 Q 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 CP 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 PQ 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 CQ 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Table 5: Confusion matrix for a held-out test set; The row label indicates the class, while the column indicates how the token was classified automatically. Tables 4 and 5 show the recall and precision for each class and a sample confusion matrix for one run of the best performing model with a held-out test-set consisting of 40 examples.Table 4 shows that the overall tendency is for both recall and precision to be higher forclasses that are more frequent, and lower for the less frequent classes as one would expect.Table 5 shows there aren’t any significant sources of confusion as the errors are spread outacross different classes. 187

Jordan & Walker 6. Discussion and Future Work This article reports experimental results for training a generator to learn which attributesof a discourse entity to include in an object description. To our knowledge, this is thefirst reported experiment of a trainable content selection component for object descriptiongeneration in dialogue. A unique feature of this study is the use of theoretical work incognitive science on how speakers select the content of an object description. The theories weused to inspire the development of features for the machine learner were based on Brennanand Clark’s (1996) model, Dale and Reiter’s (1995) model and Jordan’s (2000b) model.Because Dale and Reiter’s model relies on a model of discourse structure, we developedfeatures to represent Grosz and Sidner’s (1986) model of discourse structure, as well asfeatures representing two simple recency based models of discourse structure. The objectdescription generators are trained on the coconut corpus of task-oriented dialogues. Theresults show that: • The best performing learned object description generator achieves a 60% match to human performance as opposed to a 17% majority class baseline; • The assumed familiarity feature set improves performance over the baseline; • Features specific to the task, speaker and discourse entity (the inherent feature set) provide significant performance improvements; • The conceptual pact feature set developed to approximate Brennan and Clark’s model of object description generation significantly improves performance over boththe baseline and assumed familiarity; • The contrast set features developed to approximate Dale and Reiter’s model sig- nificantly improve performance over both the baseline and assumed familiarity; • The intentional influences features developed to approximate Jordan’s model are the best performing theoretically-inspired feature set when taken alone, and thecombination of the intentional influences features with the contrast set fea-tures is the best performing of the theoretically-based models. This combined modelachieves an accuracy of 46.6% as an exact match to human performance and holdsmore promise of being general across domains and tasks than those that include theinherent features. • Tests using singleton feature sets from each model showed that frequency features and the attributes last used have the most impact in the conceptual pact model, thedistractor set features are the most important for the incremental models, and fea-tures related to state have the biggest impact in the intentional influences model.But none of these singleton features perform as well as the feature combinations inthe related model. • A model consisting of a combination of the best singleton features from each of the other models was not significantly different from the best learned object descriptiongenerator and achieved a 53% match to human performance with the advantage offewer situation specific features. 188

Learning Content Selection Rules for Generating Object Descriptions Thus the choice to use theoretically inspired features is validated, in the sense that every set of cognitive features improves performance over the baseline. In previous work, we presented results from a similar set of experiments, but the best model for object description generation only achieved an accuracy of 50% (Jordan & Walker,2000). The accuracy improvements reported here are due to a number of new features thatwe derived from the corpus, as well as a modification of the machine learning algorithm torespect the fact that the training data for these experiments is not noisy. It is hard to say how good our best-performing accuracy of 60% actually is as this is one of the first studies of this kind. There are several issues to consider. First, the objectdescriptions in the corpus may represent just one way to describe the entity at that point inthe dialogue, so that using human performance as a standard against which to evaluate thelearned object description generators provides an overly rigorous test (Oberlander, 1998;Reiter, 2002). Furthermore, we do not know whether humans would produce identical objectdescriptions given the same discourse situation. A previous study of anaphor generationin Chinese showed that rates of match for human speakers averaged 74% for that problem(Yeh & Mellish, 1997), and our results are comparable to that. Furthermore, the resultsshow that including features specific to speaker and attribute values improves performancesignificantly. Our conclusion is that it may be important to quantify the best performancethat a human could achieve at matching the object descriptions in the corpus, given thecomplete discourse context and the identity of the referent. In addition, the difficulty ofthis problem depends on the number of attributes available for describing an object in thedomain; the object description generator has to correctly make four different decisions toachieve an exact match to human performance. Since the coconut corpus is publiclyavailable, we hope that other researchers will improve on our results. Another issue that must be considered is the extent to which these experiments can be taken as a test of the theories that inspired the feature sets. There are several reasons tobe cautious in making such interpretations. First, the models were developed to explainsubsequent reference and not initial reference. Second, the feature sets cannot be claimed inany way to be complete. It is possible that other features could be developed that providea better representation of the theories. Finally, ripper is a propositional learner, andthe models of object description generation may not be representable by a propositionaltheory. For example, models of object description generation rely on a representation ofthe discourse context in the form of some type of discourse model. The features utilizedhere represent the discourse context and capture aspects of the discourse history, but theserepresentations are not as rich as those used by a rule-based implementation. However itis interesting to note that whatever limitations these models may have, the automaticallytrained models tested here perform better than the rule-based implementations of thesetheoretical models, reported by Jordan (2000b). Another issue is the extent to which these findings might generalize across domains. While this is always an issue for empirical work, one potential limitation of this study isthat Jordan’s model was explicitly developed to capture features specific to negotiationdialogues such as those in the coconut corpus. Thus, it is possible that the featuresinspired by that theory are a better fit to this data. Just as conceptual pact features areless prominent for the coconut data and that data thus inspired a new model, we expect tofind that other types of dialogue will inspire additional features and feature representations. 189

Jordan & Walker Finally, a unique contribution of this work is the experimental comparison of different representations of discourse structure for the task of object description generation. Wetested three representations of discourse structure, one represented by features derivedfrom Grosz and Sidner’s model, and two recency based representations. One of the mostsurprising results of this work is the finding that features based on Grosz and Sidner’smodel do not improve performance over extremely simple models based on recency. Thiscould be due to issues discussed by Walker (1996a), namely that human working memoryand processing limitations play a much larger role in referring expression generation andinterpretation than would be suggested by the operations of Grosz and Sidner’s focus spacemodel. However it could also be due to much more mundane reasons, namely that it is possible (again) that the feature sets are not adequate representations of the discoursestructure model differences, or that the differences we found would be statistically significantif the corpus were much larger. However, again the results on the discourse structure modeldifferences reported here confirm the findings reported by Jordan (2000b), i.e. it was alsotrue that the focus space model did not perform better than the simple recency models inJordan’s rule-based implementations. In future work, we plan to perform similar experiments on different corpora with differ- ent communications settings and problem types (e.g. planning, scheduling, designing) todetermine whether our findings are specific to the genre of dialogues that we examine here,or whether the most general models can be applied directly to a new domain. Related tothis question of generality, we have created a binary attribute inclusion model using domainindependent feature sets but do not yet have a new annotated corpus upon which to test it. Acknowledgments Thanks to William Cohen for helpful discussions on the use of ripper for this problem, and to the three anonymous reviewers who provided many helpful suggestions for improvingthe paper. References Allen, J., & Core, M. (1997). Draft of DAMSL: Dialog act markup in several layers. Cod- ing scheme developed by the MultiParty group, 1st Discourse Tagging Workshop,University of Pennsylvania, March 1996. Appelt, D. (1985a). Planning English Sentences. Studies in Natural Language Processing. Cambridge University Press. Appelt, D. E. (1985b). Some pragmatic issues in the planning of definite and indefinite noun phrases. In Proceedings of 23rd ACL, pp. 198–203. Bangalore, S., & Rambow, O. (2000). Exploiting a probabilistic hierarchical model for Generation. In COLING, pp. 42–48, Saarbucken, Germany. Brennan, S. E., & Clark, H. H. (1996). Lexical choice and conceptual pacts in conversation. Journal of Experimental Psychology: Learning, Memory And Cognition, 22, 1482–1493. 190

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