Our experimental design takes advantage of the phase-in procedure for teacher access to the certification process, and the existence of a certification queue. Rather than having teachers wait in this queue, the intervention aimed to allow all eligible but not yet certified teachers (whom we define as "target" teachers) in treatment schools to immediately access the certification process at the start of the experiment (in 2009). The experiment did not change any of the requirements of certification specified in the law and regulations, but simply allowed otherwise eligible teachers in treatment schools to enter the certification process early, rather than having to wait for a few more years. In other words, the experiment accelerated access to the certification and pay increase for teachers in treatment schools, but it did not change the underlying program in any way. The experimental protocol was implemented in close collaboration with the Ministry of National Education of the Government of Indonesia, where senior officials were committed to conducting a high-quality impact evaluation, and provided exemplary support in implementation.

We first identified a near-representative sample of 360 schools across 20 districts of Indonesia to comprise the universe of the study. We started with the 2006 national teacher census, which covered roughly 1,600,000 public primary and junior-secondary teachers across 454 districts. Districts that were too small, were too dangerous to visit, or were included in a parallel randomized evaluation were excluded,<sup>11</sup> leaving us with 383 districts in the sampling frame. These represented nearly 85% of the districts and over 90% of the population of Indonesia. From these, we randomly sampled 20 districts, stratified across the five major regions of the country, with more districts assigned to regions with a larger population. The list of districts sampled and the strata they represent are presented in Table A.1. A map of the sampled districts is presented in Figure I.<sup>12</sup>

> 11 The district sampling for the two parallel sets of randomized evaluations was conducted using the same procedures, and so the 20 districts dropped on account of not wanting spillovers between the studies were also a representative sample.  However, the second study (of a parallel initiative to set up teacher working groups) ended up not being implemented. Districts dropped for access and safety reasons had a much lower population on average.

> 12 As the scale in Figure I indicates, the east-to-west distance spanned by Indonesia is greater than that of the continental United States, and our design imposed considerable logistical complexity. However, the resulting random assignment in a near-representative sample of schools provides greater external validity to our results. See Heckman and Smith (1995) for a discussion of the threats to external validity of experiments resulting from site-selection bias in experimental studies. Allcott (2015) provides evidence of such bias. The five major regions of Indonesia and the number of districts sampled in each of them (roughly proportional to population) were Java (10), Sumatra (5), Sulawesi (2), Eastern Indonesia (2), and Kalimantan (1).

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