comparisons at baseline were difficult to establish and indicative only, while follow-up scores (and the extent to which they differed across the treatment and control groups) provided meaningful and reliable information about the potential impact of the intervention.<sup>28</sup> The Early Development Instrument (EDI) (Janus & Offord, 2007) was completed by a first grade primary school teacher<sup>29</sup> who reported information on a random sample of 20 first graders enrolled in his or her class.<sup>30</sup> While potential biases in teachers’ reporting (on the basis of socio-economic background, for example) can be a legitimate concern, the reliability and validity results of studies conducted with the EDI in diverse areas of Canada and in British Columbia (where a potential racial bias towards Aboriginal children was considered possible) dispute this contention (see a summary of these studies in Janus et al., 2007). For the purposes of this study, the EDI was translated into Portuguese, and some of the items were dropped or adapted to fit the local context.

We aggregate child development outcomes into indices and performed factor analysis when appropriate, as in the case of cognitive development and parenting. For other cases, we aggregate outcomes as in Katz et al (2001), by first subtracting the mean and dividing by the standard deviation of each outcome, and then averaging for all outcomes that compose the index.

# 5. **Estimation Strategy**

We first present intention to treat (ITT) estimates of the effect of offering preschools in treatment communities. Given that about 41% of eligible target children enrolled in preschool, the ITT estimate is a weighted average effect of enrolled and unenrolled children in treatment areas. The ITT estimate represents the relevant treatment effect from the point of view of the policy maker interested in replicating this intervention model in similar contexts, where some parents do not

> 28 The level of cultural appropriateness and complexity of specific items would have equally impacted children in the two groups. Therefore, any remaining difference can be attributed to the intervention.

> 29 In each school the survey team interviewed the principal and administered the EDI with one first grade teacher. In schools with more than one first grade teacher, the survey field supervisor selected one first grade teacher randomly. Once the teacher was selected, the supervisor randomly selected 20 first graders using a random table. The supervisor then filled in 3 questionnaires (i.e., for the first 3 first grade students) with the teacher to familiarize the teacher with the instrument. The supervisor left the 17 remaining questionnaires with the teacher, for him/her to complete, and returned within 2 weeks to collect all completed surveys.

> 30 For the EDI we observe only the subset of children who enroll and are attending primary school. Given that the preschool program had a large and significant effect on primary school enrollment in treatment communities, the composition of first graders in treatment communities changed relative to controls. Evidence from the household survey indicates that the preschool intervention led more disadvantaged children to enroll in primary school, so results of the EDI are arguably lower-bound estimates of impact given a larger proportion of disadvantaged children in treatment community primary schools. Unfortunately, we do not have detailed socio-economic and demographic data on the sample of primary school children to confirm the direction of the potential sample composition bias.

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