the median, and by union (an administrative unit).<sup>8</sup> There were two deviations from the experimental protocol. First, while programming the mobile application used for data collection, 27 villages were assigned by mistake to a treatment different from the original one. The partial re-assignment of treatments was thus due to a programming error and not to the incorrect implementation of the protocol in the field. In addition, the checks for balance in covariates are very similar based on originally assigned or actual treatment (see below). For this reason treatment status is defined as _actual_ treatment in the analysis. The second deviation is that surveyors were unable to differentiate a village from the one adjacent to it in four cases. While data were collected from households in these four villages and the ones adjacent to them, only pairs of villages could be distinguished, and both villages in each pair received the same treatment. In the statistical analysis there are thus effectively 112 clusters divided into experimental arms A (49 clusters), B (48), and C (15). For simplicity, in the rest of the paper these clusters will be referred to as ‘villages’.<sup>9</sup> It should be emphasized that, while surveyors completed a census of _wells_ in study villages, our data do not represent a census of _households_ . The choice to survey only owners—who were anyway a majority—was due to budgetary constraints, but an implication is that one cannot study whether the choice of the primary source of drinking water changed also among non-owners.

# **3 Data**

A team of testers who had at least completed secondary education was recruited locally. During the home visits when sale offers were made, testers also administered a short household baseline survey and recorded information on sales and, in case of purchase, the result of the test. Additionally, surveyors recorded GPS coordinates of all wells and noted down whether there were any visible labels attached to the well indicating the status with respect to arsenic. The baseline questionnaire included a household roster, basic questions on socio-economic status, detailed questions on the well, and a number of questions related to knowledge and practice in relation to drinking water and arsenic risk. Testers also recorded whether, according to the respondent, the arsenic status of the well water was safe, unsafe, or unknown. Information about a total of 12,606 wells was recorded and the household survey was completed for all but three well owners.

A limitation of the data is that, in case of group sales in arm B, surveyors did not keep accurate records of who belonged to which group. In other words, while the data indicate for which wells a test was purchased, one cannot study the characteristics of buyers belonging to the same group, or to what extent well sharing was actually taking place _within each group_ as a consequence of the test results.

The endline survey was completed between August 2016 and January 2017.<sup>10</sup> The average time

> 8Unions are the third smallest administrative unit in Bangladesh, and are formed by several _mouzas_ , which in turn are composed of two to three villages. The 128 study villages belong to nine unions.

> 9The Appendix shows that the randomization led to large spatial variation in treatments, see Figure A.1. Unlike

> 10the baseline survey, where the wages of surveyors and the supervisor were covered mainly from test fees, the

9