between baseline and endline surveys was 7.7 months, and 86% of households had their follow-up interview between seven and nine months after the baseline interview. During the endline survey, surveyors were instructed to return to the wells identified during the test sales and record if the household owing the well was still using it as a primary source of water for cooking and drinking. In case of a negative answer, the surveyor asked the respondent to accompany him/her to the actual source and would record the new GPS location and the presence of any visible indicators of arsenic status (for instance one of the metal placards distributed during our intervention). The surveyor would also ask the respondent the perceived safety of the new source as well as about the primary reason for switching to a different source. Switching behavior was thus self-reported, but earlier work in the neighboring Araihazar sub-district found that switching behavior recorded in a way similar to this study was actually consistent with urinary arsenic concentrations, an objective biomarker of exposure (Chen et al. 2007). Unfortunately, the records on the location of the new source of drinking water do not allow to measure precisely to what extent switching was associated to a reduction in arsenic risk. The smartphone’s GPS sensor—with a precision of 10 m at best—cannot uniquely identify a specific well among those surveyed due to the density of wells within a typical village in Bangladesh. The GPS data was still used to estimate the distance from the old to the new well used for drinking.

# **3.1 Summary Statistics at Baseline**

Table 1 shows selected summary statistics measured at baseline. Throughout this paper, unless otherwise noted, the analysis is restricted to the large majority of households (91%) that used their own well at baseline, as this is the sample for which baseline water source and post-intervention switching can be determined. The baseline source was not recorded for non-users, and during the endline survey they were only asked again if they used their own well for drinking and cooking.<sup>11</sup> All summary statistics except those on the first row of Table 1 are thus calculated for households who used water from their own well for drinking and cooking.

Household heads had low levels of educational attainment on average, and most households were poor, with only 17% of the houses having a concrete roof (an indicator of wealth), while the rest had tin or (in rare cases) mud roofs.

The average well owner in our study area lived in a village where 75% of the wells tested by BAMWSP between 1999 and 2000 were unsafe with respect to arsenic. Despite the BAMWSP blanket testing campaign, a large majority of respondents (76%) did not know whether their well was safe or unsafe with respect to arsenic. Only 7% of them thought that their well was unsafe, while the remaining 17% reported believing that their well was safe. Whereas about a quarter of respondents indicated that they knew the status of their well, more than 99% of wells lacked any visible sign of their status with respect to arsenic such as red or green paint. Information on the safety of the minority of wells that had been tested was thus not immediately observable by other households, although in

> cost of the follow-up survey was paid for by the project.

> 11Wells not used for drinking were on average significantly shallower, and thus more likely to be contaminated with high levels of arsenic. Of the 1,193 wells not used for drinking, only 14 (about 1%) were believed to be safe by the owner.

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