village children aged seven years or less and all pregnant and lactating women. Cohorts of newborns were included until September 1977. Data collection for individual children ceased when they reached seven years of age. The birth years of children included in the 1969-77 longitudinal data collection thus range from 1962 to 1977, so when the intervention ended, their ages ranged from 0 to 15 years.<sup>8</sup> Therefore the length and timing of exposure to the nutritional interventions (described below) for particular children depended on their respective birth dates. For example, only children born after mid-1968 and before October 1974 were exposed to the nutritional intervention for all of the time they were from six to 36 months of age, which often is posited to be a critical time period for child growth in the nutrition literature (see Maluccio _et al._ 2005, Martorell, Habicht and Rivera 1995 and Martorell _et al._ 2005 and the references therein).

The principal hypothesis underlying the intervention was that improved pre-school nutrition accelerates physical growth and mental development. To test this hypothesis, 300 villages were screened to identify those of appropriate size, compactness (so as to facilitate access to feeding stations, health centers and psychological testing sites, see below), ethnicity, diet, educational levels, demographic characteristics, nutritional status and degree of physical isolation. From this screening, village pairs similar in these characteristics were determined: Conacaste and Santo Domingo (relatively populous villages) and San Juan and Espíritu Santo (relatively less populous villages).

Two villages, Conacaste and San Juan, were randomly assigned to receive a high protein-energy drink, _Atole_ , as a dietary supplement. _Atole_ contained Incaparina (a vegetable protein mixture developed by INCAP), dry skim milk, and sugar and had 163 kcal and 11.5 g of protein per 180 ml cup. This design reflected the prevailing view of the 1960's that protein was the critically limiting nutrient in most developing countries. _Atole_ , the Guatemalan name for hot maize gruel, was served hot; it was pale gray-green and slightly gritty, but with a sweet taste.

In designing the data collection, there was considerable concern that the social stimulation associated with attending feeding centers—such as the observation of children’s nutritional status, the monitoring of their intakes of _Atole_ and so on—also might affect child nutritional outcomes, thus confounding efforts to understand the impact of the supplement. To address this concern, in the remaining villages, Santo Domingo and Espíritu Santo, an alternative drink, _Fresco_ , was provided. _Fresco_ was a cool, clear-colored, fruit-flavored drink. It contained no protein and only sufficient sugar and flavoring agents for palatability. It contained fewer calories per cup (59 kcal/180 ml) than _Atole._ Several micronutrients were added to the _Atole_ and _Fresco_ in amounts that achieved equal concentrations per unit volume. This was done to sharpen the contrast between the drinks to protein; the energy content differed, of course, but this was not recognized to be of importance at the time.

The nutritional supplements (i.e., _Atole_ or _Fresco_ ) were distributed in supplementation centers and were available daily, on a voluntary basis, to all members of the community during times that were convenient to mothers and children but that did not interfere with usual meal times. For this study, with the differential “intent to treat” exposure to these nutritional supplements as first-stage instruments to estimate relation (2), a critical question is to what extent the intervention design

> 8 Though the number of villages is small, the number of village-birth-year cohorts is 64, which provides a reasonable number of clusters for estimation of standard errors that account for clustering.  Maluccio _et al._ (2005a) and Behrman _et al._ (2005) provide such estimates and report that they do not differ much from estimates with robust standard errors as are used in the present study.

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