Table 2 gives an overview of data collection activities and timing of the preschool construction. Our analysis is based on three main waves of data collection: a baseline data collection in May—July 2016, a midline survey in April—June 2017 , and an endline survey in May—July 2018.<sup>16</sup> With 82% of CPS completed for the first school entry in October 2016<sup>17</sup> and 91% of CPS constructed at endline, Table 2 confirms the construction rolled out as planned. Nevertheless, and despite our effort to ensure that the baseline survey preceded CPS construction, a completed CPS building was already available at baseline in 17% of the treatment group villages. It is challenging to conduct an experiment like this with school construction. On the one hand, fielding the baseline too early (well before any construction) would have increased the risk (in case of construction delay) that our baseline sampled children would have been too old to attend the newly built preschools.<sup>18</sup> On the other hand, fielding the baseline too late would have resulted in baseline measures that were arguably already affected by the program. We discuss the implications of the slight overlap between baseline survey and construction below.

During baseline data-collection in 2016, we sampled up to 26 eligible households per village, using an adapted version of the EPI walk,<sup>19</sup> which guarantees representativity of households with children of preschool age.<sup>20</sup> Eligible households include at least one child between 24 and 59 months old at baseline. Children were therefore between three and five years old at midline and between four and six years old at endline. We identified 7053 eligible households at baseline. Using the same sampling method, we added extra households at midline in villages where the number of eligible households at baseline was below 10 (53 were added in total).

# **3.2 Data**

## **3.2.1 Survey and instruments**

For each household, a household survey, caregiver survey, and one assessment per eligible child were conducted.<sup>21</sup> At the village level, interviews were conducted with village heads and preschool teachers. In addition, at endline, we complemented the

> 16A brief monitoring survey was conducted in late 2016 to confirm that CPS construction was proceeding as scheduled.

> 1717 CPS were in construction at that date. 10 of them would be completed by the end of the calendar year 2016. It further means that the disturbance caused by the construction is probably minimal

> 18As described in Bouguen et al. (2013), construction delays occurred in a previously evaluated program in Cambodia. This considerably reduced take-up, exposure time, and statistical precision.

> 19EPI refers to the Expanded Programme on Immunization of the World Health Organization; see e.g., Henderson and Sundaresan (1982).

> 20The sample is not nationally representative. Villages were selected based on criteria such as expressed interest in the program, poverty rate, lack of a functioning preschool, and teacher availability. Sample households are exclusively from rural areas in southern and eastern Cambodia. This is because western provinces received CPS under a previous preschool construction program.

> 21The caregiver is defined as the direct relative who takes care of the child most of the time. In most cases, the caregiver is a biological parent (60.4% at baseline).

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