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close this book Food Composition Data: A User's Perspective (1987)
close this folder Other considerations
close this folder A system for evaluating the quality of published nutrient data: Selenium, a test case
View the document (introductory text)
View the document Introduction
View the document Background
View the document Procedure
View the document Criteria
View the document Calculation of the mean SE value and confidence code
View the document Results
View the document Discussion
View the document Implications
View the document Acknowledgements
View the document Disclaimer
View the document References

Procedure

Procedure



Fig. 1. Flowchart of evaluation system for published selenium data.

The system of objective criteria described here for the evaluation of published nutrient composition data reflects the basic concepts described by Exler [8] and incorporates the critical aspects cited by Stewart [26]. The system involves several stages (fig. 1): (a) development of categories of criteria to include key issues relative to analytical methodology and sampling; (b) review of literature and collection of those papers that reported the Se content of foods and related methodology; (c) definition of criteria in each category at four rating levels to reflect specific concerns that pertain to the evaluation of literature reporting the Se content of foods; (d) assignment of ratings for each criterion by study and by food item; (e) derivation of the quality index for each study; and (f) derivation of mean value and CC for the Se content in each food by combining data from the acceptable studies.

Five general categories were developed to evaluate the data for each food item: (a) number of samples, (b) analytical method, (c) sampling handling, (d) sampling plan, and (e) analytical quality control. These categories identify the issues essential to any study related to the composition of foods. A rating scale of 0 (unacceptable) to 3 (most desirable) was established. Within each category, the criteria for each rating level were defined specifically for Se. Establishing these criteria demanded knowledge of accepted methodology, sample handling procedures, and quality-control measures specific to this nutrient. In addition, knowledge of statistical methods was required for the sampling plan and number of samples categories. The rating criteria are outlined in table 1 and described in detail below. In general, within each category the level of documentation and appropriateness of procedures are addressed.

An extensive literature search yielded approximately 65 papers (from 33 different journals, reports, proceedings, and books) published after 1960 that report original analytical Se food data. Several references include data from more than one study. Papers published prior to 1960 were collected for historical purposes, but the data were not included in this evaluation because of difficulty in assessing the validity of the methodology used and conceivable lack of relevance of those data to current studies of food consumption due to possible changes in the food product during the last 25 years. The methodology papers referenced in the data articles were also collected. In addition, data from recent FDA Total Diet Study analyses, unpublished at the time of this evaluation [23], were included because this programme is one of the few that has analysed food as eaten, i.e. cooked foods and mixtures. Data from fiscal years 1982/83 and 1983/84 were treated separately, providing two sets of means for each food item analysed. Since the focus of this work was the Se content of foods frequently consumed by Americans, only those studies that analysed foods grown, processed, or sold in the United States and Canada were collected.

As previously mentioned, the data from each study were rated on the criteria on a scale of 0 to 3. In general, O was assigned when information was not adequate to permit evaluation of data for use in food composition data bases, or when certain procedures or practices were inappropriate. A 3 was assigned when procedures were well-documented and appropriately applied.