students substituted solar lamps for kerosene lamps almost completely, in order to study at night. Moreover, there was no significant difference in the treatment effects on children who received three solar products and those who received only one solar lantern.<sup>15</sup> Learning is a process of accumulating knowledge. Compared with kerosene-based products (that rely on expensive fuels), solar lanterns also provide reliable lighting conditions that supposedly make learning more productive. Considering these possibilities, the identified increase in home study time must not be undervalued.

Figure 1 plots the estimated monthly treatment effects (with 95% confidence intervals) on daily time use during the eight months. Home-study time increased by approximately 20 hours in November, just before the annual examination season (December) in Bangladesh. On the other hand, the time spent on other activities (sports, cooking and bathing, household chores, sleeping and rest, and other activities) declined during that month. This increase in home-study time in November is not negligible, given that the control students spent approximately 120 hours, as a mean, studying at home that month; students intensively prepare for an examination in November in order to move up to the next grade.

One may challenge the accuracy of the students’ self-reported time use, given the potential for measurement errors. However, the distinct time-variation associated with the treatment effects helps to alleviate this concern.<sup>16</sup> If the self-reported study hours were entirely noise, it then becomes difficult to explain why they increased just before the examination season. While the respondents may have over-reported their nighttime study hours, for reasons of social desirability bias, there is no a priori reason why such a response bias should have led to a reduction in the number of daytime study hours reported, as seen in columns (c) and (i) in Table 5. Moreover, potential measurement errors in the dependent variables would have affected the efficiency of the estimates, and not (as long as they were classical) their consistency.<sup>17</sup> The research team made surprise visits to catchment schools in February, April, and August 2014, and checked on the children’s school attendance. The estimated impacts are reported in columns (a)―(d) (with no baseline controls) and (e)―(h) (with controls) in Table 6. As the results indicate, treated students who received three solar lamps (Treatment A, all products) attended school more regularly, as revealed during our first two visits in February and April. However, this positive impact did not last long; it became statistically insignificant in August 2014. In columns

> 15In Table S.13 in the supplemental appendix, we have disaggregated the treatment effects on student time spent on “other activities” (estimated in columns (f) and (l) in Table 5) into “sports,” “cooking and bathing,” “household chores,” and “other activities, including sleeping and rest.” We obtained some (albeit statistically insignificant) evidence that suggests treated students reduced the time they spent on “other activities, including sleeping and rest.”

> 16Instead of using a time diary, we could have employed a more sophisticated sensor to measure the students’ time use. However, such a device would not necessarily have been more accurate than the time diary in measuring the number of hours the students used to study. There would still have been a concern about the accuracy of measurements unless the research team had been able to monitor the students’ use of the sensor; that would have required precise (and practically impossible) monitoring of the students’ behavior during the experiment.

> 17By treatment arms, Table S.14 in the supplemental appendix provides similar information to that reported in Table 1, collected during a follow-up household survey. As seen in Panel D of Table S.14 (presenting the proportion of respondents who used each light source to enable activities through multiple answers), approximately 73% (100%) of households that received three (one) solar products used S250 for reading and studying. Interestingly, compared to households that received three products, households that received only S250 used this flagship lamp more intensively for reading and studying.

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