demographic and socio-economic characteristics are also similar across treatment and control groups. The baseline report provides tests of balance on a more extensive set of variables.<sup>6</sup>

# _6.2 Attrition in the Follow-up Data_

A legitimate concern in any follow-up survey involves the extent of sample attrition and the degree to which attrition is nonrandom. In the baseline, a total of 2,087 households were interviewed. Two years later, 1,908 households were successfully re-interviewed. Of the original 2,087 baseline household, 179 could not be contacted (86 households in the control group and 93 households in the treatment group). This figure translates into an overall attrition rate of 8.5 percent.

Despite the low rate of attrition found in the TSSM sample, nonrandom attrition could still lead to biased inferences. In this section, we investigate nonrandom attrition by examining the characteristics of households that left the sample. We examine whether means of outcome and control variables for the attriting households differ across treatment and control communities.

As Table 3 shows, the attriting household characteristics are similar across treatment and control groups. Only one difference is statistically significant at the 5 percent level: the attriting treatment group has fewer household members under five years of age than the attriting control group. A small number of differences are significant at the 10 percent level: the percentage of household heads with senior high school education is higher in the attriting treatment group ( _p=_ 0.057), as is dwelling ownership ( _p=_ 0.09) and the use of concrete walls ( _p=_ 0.05). All of these differences are relatively small and based on a small number of observations.

We also examine whether the households that dropped out of the sample differ from those that were found and surveyed for the endline. Table 4 suggests that the attriting households and panel households differ in some respects. Attriting households were less likely to have owned their dwelling at the baseline (73 versus 83 percent, _p=_ 0.001). They were also more likely to have a female head (10.6 versus 3.9 percent, _p<_ 0.000), and have more adults ( _p=_ 0.004) and fewer children between ages six and 10 (but no difference in the number of children under age two). Attriting households were also slightly wealthier ( _p=_ 0.098) and used more liquid petroleum gas (LPG) as cooking fuel ( _p<_ 0.05) _._ As discussed earlier, we added households to the endline sample to attain a sample size of 2,500 households. We did this to increase the power of our endline tests. Table 4 further examines whether the added households differ from the panel households. Columns 3 – 5 report the results of tests of difference in means. Only a small number of differences are statistically significant at the 10 percent level. A lower percentage of the new endline households (20 percent) received

> 6 See Annex 1 in _Scaling Up Rural Sanitation: Findings from the Impact Evaluation Baseline Survey in Indonesia_ , Water and Sanitation Project Technical Paper, November 2010; http://water.worldbank.org/publications/scalingrural-sanitation-findings-impact-evaluation-baseline-survey-indonesia.

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