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close this bookEarly Supplementary Feeding and Cognition (Society for Research in Child Development, 1993, 123 pages)
close this folderV. Methods of the cross-sectional follow-up
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Socioeconomic indicators

Census data from each of the four villages were collected prior to the initiation of the study (1967), in the midst of the supplementation period (1974), and prior to the follow-up (1987). From these data, three indicators of socioeconomic status (SES) that reflect family wealth and potential for child stimulation were constructed: house quality, mothers' education, and fathers' occupation. Father's education was not included because the informants (mothers) often lacked knowledge about their husbands' schooling. In the final analyses, these indicators were standardized and summed to yield a single composite socioeconomic index.


FIG. 4. - Breakdown of the follow-up sample by age and duration of exposure

For each census, all women over the age of 15 who had ever been married and all mothers in each village were interviewed, and information was obtained on each member of the nuclear family living in the household. The census forms for 1974 and 1987 were identical; the one used in 1967 was somewhat different but yielded comparable information. The informants also provided information about family structure, marital status, religion, number of pregnancies, number of live children, and relationship of household head to head of the extended family. Observations of the quality of the house were also made (e.g., type of walls, floor, and roof). For each family member, the parity, relationship to the rest of the family, birth date, education, occupation (if over age 10), and date of change of status (death or migration) were coded. In addition, in 1987, when many of the subjects had started their own families, information about their families of origin was also recorded. Table 9 shows the number of families per village that were included in each round of the census as well as the total number of families for whom data were obtained at least one of the three assessments.

TABLE 9: NUMBER OF FAMILIES WITH SES DATA BY YEAR OF CENSUS AND VILLAGE


1967

1974

1987

Total a

Fresco 03

174

225

369

471

Atole 06

178

236

368

464

Fresco 08

132

187

254

325

Atole 14

110

136

237

281

NOTE - Villages are identified by census codes.

a Number of families for whom data were obtained at any assessment date; new families formed in 1987 are included.

Construction of SES Measures

House Quality

In developing countries, it is often difficult to obtain accurate information on family income, a variable that traditionally has served as a proxy for a number of social-environmental variables that affect cognitive growth and educational development. This is particularly the case in rural agricultural communities and explains why indicators of house quality have often been used as proxies for income in such communities (e.g., Johnston, Low, de Baessa, & MacVean, 1987).

Nine variables describing house quality were assessed at each of the three census periods: an overall rating of the type of house (on a scale of 1-4); ownership of house (no = 0, yes = 1); number of rooms; type of floor (1-5); type of walls (1-7); type of roof (1-4); location of the kitchen (1-3); type of toilet (1-4); and number of possessions (1-6). In all instances, higher scale scores reflected the higher quality of the dwelling. Data were also available for type of water disposal, source of water, and presence of electricity. However, these variables were not used in the assessment of house quality because they tended to be village specific and contributed little to within-village variation.

To generate an index of within-village variation in house quality and of socially meaningful between-village differences, a factor analysis was performed to generate factor loadings for each assessment year. The nine variables were standardized within each village to allow for comparability across villages and then factor analyzed at each year using a principal components analysis. The results are shown in Table 10. Since the second principal component did not account for more than 13% of the variance at any year, it was dropped from further analyses; loadings on the first factor were similar at each time period.

In order to recapture between-village differences that were removed through standardization, final house quality scores were constructed by multiplying the original individual raw scores by the factor loadings. An alternative approach, used in Ruel (1991), eliminated variables that varied systematically by village and derived the factor scores without multiplying the raw scores by the factor loadings. These scores, which were calculated for the 1974 and 1988 data, correlated at r =.91 and .88, respectively, with the scores that we used for analyses reported here.

TABLE 10: FACTOR LOADINGS ON THE FIRST PRINCIPAL COMPONENTS FACTOR OF THE NINE INDICES OF HOUSE QUALITY (Using within-Village Standardized Scores)

Variable

1967

1974

1987

Possessions

.50

.53

.60

House

.84

.88

.88

Ownership

-.03

-.01

-.04

Rooms

.77

.74

.62

Floor

.51

.65

.82

Walls

.80

.83

.82

Roof

.52

.54

.56

Toilet

.32

.46

.55

Kitchen

.71

.64

.29

Eigenvalue

3.32

3.66

3.57

% variance

.37

.41

.40

Parents' Education

Parents' education has been consistently shown to be positively related to the cognitive development of the offspring (Sigman, Neumann, Jansen, & Bwibo, 1989). The mechanisms underlying this close-to-universal association are numerous, and they probably vary across cultures. In a variety of studies, parents' education has been positively related to frequency of educational opportunities available to the children, verbal stimulation provided to the offspring, and parents' aspirations for their children (Levine et al., 1991; Sigman et al., 1988; Sigman et al., 1989).

Informants reported parents' literacy (coded 0 = none, 1 = some) at all three census periods and parents' years of schooling in 1974 and 1987. The mean number of years of schooling in 1974 and 1987 was, respectively, .98 and 2.1 for mothers and 1.3 and 2.5 for fathers.

Parents' Occupation

Occupational status is a "carrier" variable that may be associated with income status in the community. availability of resources. and family socialization practices. The indirect effects of parents' occupation on the cognitive development of children are thought to occur through the earning capacity of the parents and the consequent resources for stimulation that that earning capacity permits. Both mother's occupation and father's occupation were assessed; however, since only about 20% of women at the follow-up reported having an occupation, mother's occupation was excluded from further analyses.

TABLE 11: CORRELATION OF SES MEASURES OVER THE THREE CENSUS PERIODS


1967-1974

1967-1987

1974-1987

House factor

.69**

.37**

.50**


(466)

(360)

(533)

Mother:


Literacy

.72**

.63**

.77**



(429)

(326)

(491)


Education a



.80**





(488)

Father:


Literacy

.82**

.82**

.77**



(380)

(282)

(433)


Education a



.73**





(397)


Occupation

.36**

.27**

.37**



(371)

(282)

(427)

NOTE - Pearson product-moment correlation coefficients were computed for house quality and education and Spearman rank-order coefficients for literacy and occupation. Sample size is given in parentheses.

a Data are not available for 1967.

**p <.01.

There were six occupational categories listed in the 1967 census and 19 in the 1974 and 1987 censuses. For purposes of comparisons across years, the original 0-19 scales used in 1974 and 1987 were collapsed to be similar to the 1967 scale; these original and recoded scales are highly correlated (r =.88). In preliminary analyses, the recoded scale demonstrated adequate linear properties and was used in all subsequent statistical calculations as an ordinal variable.

Reliability of Measures

As shown in Table 11, most indicators remained stable over time. As expected, the highest correlations are obtained for variables that have little intraindividual variability and little expected change, such as years of school attainment, while the smallest values occur for variables that might be more likely to change over time, such as the characteristics of the family's home.

In subsequent, final analyses, indicators obtained in 1987 were used. An SES composite was created by summing three standardized variables: house factor score, father's occupation, and mother's schooling. As noted earlier, mother's occupation was dropped owing to its low occurrence, and father's education (as reported by the mother) was also excluded since the mothers often could not provide accurate information.