|Early Supplementary Feeding and Cognition (Society for Research in Child Development, 1993, 123 pages)|
|V. Methods of the cross-sectional follow-up|
As a result of a 1987 census, 1,704 subjects were identified as composing the potential sample for follow-up assessment in the behavioral area. Owing to the large number of outcome variables of interest and the amount of time needed to test each subject, the cohorts born prior to 1965 were excluded from this aspect of the follow-up study. The intent was to maximize the amount of information to be collected without loss of the theoretically most important cohorts; since cohorts born between 1962 and 1965 had received supplementation at a noncritical developmental period (age 4-7), these were considered to be of least theoretical interest.
Of the 1,704 subjects, 1,545 were residing in the villages, and, of those, 93% completed the battery of psychoeducational tests that were selected for the follow-up assessment. With the inclusion of individuals who had migrated out of the villages but who could be contacted for testing either in Guatemala City or in surrounding villages, coverage of all potential subjects decreased to approximately 83%. Additional data cleaning resulted in the elimination of approximately 30 subjects from the psychology battery. A breakdown of the final number of subjects available for the follow-up psychoeducational tests is presented in Figure 3.
Comparisons of participants and nonparticipants allow us to make some inferences regarding the representativeness of the follow-up sample. Participants in the follow-up had higher mean birth weights, were less frequently ill with diarrhea from birth to 3 years of life, and had higher average energy intakes from the supplement during the first 3 years of life than nonparticipants (Rivera & Castro, 1990). However, this difference existed in both the Atole and the Fresco villages, and the rate of participation among village residents was similar in both types of sites (94% and 93% for females and 86% and 84% for males in the Atole and Fresco villages, respectively).
In addition, the percentage of migrants - defined as subjects not living in the villages at the time of the follow-up - from the entire sample was also similar in the Atole (32.7%, N = 377) and the Fresco (34.4%, N = 350) sites (Rivera & Castro, 1990), as was the proportion of migrants who participated in the follow-up (41.4% in the Fresco and 40.0% in the Atole villages).
Comparisons of the results as a function of migratory status, and with migrants removed from analyses, yielded two important pieces of information. First, although the migrants who enlisted in the follow-up performed significantly better than the nonmigrants on all psychoeducational tests, their performance was similar across Atole and Fresco subgroups. The only exception occurred on the Raven's Progressive Matrices test: migrants who were natives of the Fresco villages performed significantly better on this test than those from the Atole villages. When migrants were removed from the main analyses, the results of these remained virtually unchanged. Thus, although the follow-up sample may have been slightly better off than the entire longitudinal sample and migrants may be better off than nonmigrants, none of the observed sampling differences should modify any observed treatment effects.
Ideally, the entire sample would be broken down by cohorts defined by periods of exposure (e.g., prenatal only, postnatal only, first 2 years of life) so as to assess whether developmental period and duration of exposure modified the effects of the nutrient supplement; however, restrictions of statistical power precluded such an approach. 6 Accordingly, our analyses focus on the entire sample and on a cohort of maximum exposure, that is, subjects who were exposed to the treatment during gestation and for at least the first 2 years of postnatal life. Sample sizes in these two cohorts are large, with power to detect even small effects (power =.98 for an effect size greater than .20). In addition, the final analyses included a "late" cohort whose age at first exposure was 24 months or older (power =.74 for an effect size of .25).
6 For subjects with prenatal exposure only,
sample size was 77 (52 for tests requiring literacy). Estimated power for an
effect size of .25 was 58 (42 for literate group). For subjects with postnatal
exposure only, sample size was 30. Estimated power for effect size of .25 was
The composition of the follow-up sample according to age of exposure and its duration is presented diagrammatically in Figure 4. The cohort of maximum exposure includes all subjects who received nutritional treatment during a period of accelerated brain growth, which represents a sensitive, if not the most sensitive, period when considering the effects of a nutritional intervention. These subjects were born between 1970 and 1974 and ranged in age from 13 to 19 years at the time of the follow-up study. They represent the most suitable sample on which to test the effects of the treatment. By contrast, the cohort of late exposure, born during or prior to 1967, helps determine whether exposure during the period of accelerated brain growth is a necessary condition for treatment effects.