PROSPERA brings children into school who might have otherwise stayed home or engaged in work. These children may be more likely to be in the lower part of the test score distribution. If selective school attendance is not taken into account, then estimated program impacts on test scores are likely to be downward biased.

A related caveat is that little is known about how impacts might vary by available school quality. Maintaining and/or improving the supply of schools and their quality are responsibilities of the Ministry of Public Education in Mexico. However, PROSPERA impacts in the area of education are intimately intertwined with school quality. First, schooling quality may, in fact, have deteriorated for schools where PROSPERA beneficiaries attend due to crowding induced by increased enrollment. Second, the extent of benefits accruing to children from attending school may depend on school quality. Third, changes in school quality may affect decisions about which schools to attend, which is more likely to be the case in urban areas and at higher school grade levels where households can choose among multiple schooling options.<sup>3</sup> This study analyzes nationally applied achievement test data that have recently been linked to administrative data on beneficiary receipt, socio-economic characteristics of children taking the tests and school quality characteristics to study how the PROSPERA program affects school enrollment, type of school attended and academic achievement. Nationwide standardized administrative test score data from schools (ENLACE National Evaluation of Academic Achievement in Schools “Evaluacion Nacional de Logro Academico en Centros Escolares”) were merged with information on which students come from PROSPERA households and with contextual information on family background obtained from a school survey. These data allow study of how students from beneficiary and non-beneficiary households differ in terms of their enrollment decisions, their grade progression and in terms of learning and academic achievement, as measured on standardized tests in math and Spanish.

As previously noted, a major challenge in assessing program effects on learning and academic achievement is the problem of selection. In primary school (up through grade 6), attendance at school is nearly universal, but in subsequent grades students or their families decide both whether and where to attend school and what type of school to attend. There is also selection into testtaking, as the achievement tests available are only administered to students who are enrolled in school. Simple comparisons of mean test scores of students from beneficiary and non-beneficiary households might show that students from beneficiary households have lower scores, simply because weaker students are induced by the program to enroll in school and thus take the standardized achievement tests. In making comparisons between children from beneficiary and non-beneficiary households, one needs to account for how the program affects the composition of students present in school. The selection problem is dynamic as it occurs at each grade level

> 3 We do not attempt a comprehensive definition of school quality but rather are limited to available information in our data sources. By school quality in this study we refer to school level characteristics which reflect available school resources including infrastructure, supplies and teacher characteristics and, given high inequalities in these variables nationwide, are presumably indicative of differences in resource allocation.

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