15 Our interventions were implemented during the July 2014 billing cycle in Belén, when households received water bills that were based on their water consumption in a 31-day period prior to this bill being generated.

We call our post-intervention consumption variables ‘August 2014 Billed Consumption’ and ‘September 2014 Billed Consumption’.<sup>9</sup> As noted above, the way that bills are generated and distributed means that each bill covers the 31-day period prior to its generation. This means that consumption in an August bill, for example, covers a month-long period which lies mostly in July and partly in August - but all of which lies after the consumption period covered by the July bill. Thus, our outcome data is water consumption for each household _during a two-month period since they received the intervention bill_ . For the bulk of our analysis, we will report results on the average post-intervention water consumption, i.e. the mean of August and September 2014 billed consumption.

We rely throughout on water consumption in the previous wet or rainy season as our main measure of counterfactual water consumption, i.e. a measure of how much water the households in our experimental sample would have consumed, absent an intervention. The rainy season runs from May through November in Costa Rica, and thus covers both our implementation and post-intervention periods. We use this in preference to using the previous months of consumption in 2014 because several of them would fall in the dry season, when water use patterns are likely to have been quite different from those in the post-intervention months for which we have data.

**Table 1** compares baseline water consumption in our 4 intervention (3 treatment and one control group) arms to check for the validity of our randomization. As it shows, there was no significant difference in baseline water consumption (which we measure baseline consumption using all available data on water consumption in 2013<sup>10</sup> ) between households in any of our treatment groups and the control group. We have also computed year-on-year increase in water consumption between the two months prior to the intervention and the corresponding two months of the previous year, and find no significant differences. After confirming that our treatment and control groups are identical with respect to past water consumption, we turn to the measurement of the impact of our interventions.

# **VI: Results**

# **_Result 1: Treatment groups reduce water consumption more than the control group_** _._

**Table 2** shows the differences between average post-intervention water consumption (i.e. the average of August 2014 Billed Consumption and September 2014 Billed Consumption) and the average water

9 As discussed above, the system of billing in Belén means that the exact dates of each billing cycle (and thus the precise timing of water consumption which the bill is based on) vary by postal route. For simplicity, we refer in this paper and tables to the water consumption on which the bill for month X is based “Month X Billed Consumption”, noting that it in fact refers to the consumption over a 31-day period prior to the generation of the bill for Month X. 10 Note that the municipality did not have data for December 2013. We have therefore used January-November 2013 and December 2012-November 2013 as our “baseline period” for the purposes of the randomization check.