They are 0.06 for the effect at the control mean and 0.04 for the interaction effect; only the first
remains statistically significant at conventional levels (5 percent level). It may be that stronger
students were able to utilize the supplied books independently of the teacher than weaker
students, and as a result, they benefited more than their classmates from the opportunity to read
in school. This is consistent with other studies that have observed that stronger students tend to
experience larger treatment effects from self-directed interventions (e.g. He, MacLeod, and
Linden, 2008).
Finally, Table 6 disaggregates the effect on the reading test by competency. The first two
columns report the results from the first follow-up survey, while the last two columns report the
results from the second. The first column for each of the follow-up surveys provides the
differences between the average test scores of the control and treatment groups using equation
(1). The second column provides the estimates with the full set of controls using equation (2).
Finally, the last two rows of Table 6 report the chi-squared statistic and p-value from a test of the
joint significance of the coefficients on the treatment indicator from the regressions for the
individual components of the reading test, estimated using seemingly unrelated regressions.
On both the first and second follow-up surveys, the program had a positive effect on most
of the components on the test (the written part was the one exception). In the first follow-up
survey, there are sizable treatment effects on sound recognition, word recognition, and the ability
to answer questions from the oral reading passage. The effects on word recognition and the oral
questions are individually statistically significant at the one and 10 percent levels respectively,
but the effect on sound recognition is not statistically significant at conventional levels. The joint
test of an effect on all skills shows that the results for the different components are jointly
statistically significant with a p-value of 0.013.