A large set of studies strongly supported the effectiveness of water treatment (i) on‐site,<sup>8</sup> (ii) across the supply, and (iii) at the source in reducing children’s exposure to diarrhea. Other studies indicated the effectiveness of combining water quality and hygiene interventions (including the provision of soap for handwashing and hygiene information campaigns) in reducing several diarrhea‐related indicators (e.g. rates of incidence, episodes per person, and prevalence).

In general, impact evaluations that included a large number of studies as well as those that applied more rigorous research methods had less ambiguous results. Specifically, individual RCTs reported significant reductions in dysentery, influenza, shigellosis, conjunctivitis, respiratory diseases, parasite infections, and impetigo. Water supply interventions also produced strong reductions in enteric and other health‐related diseases. Conversely, impact evaluations that included only a small number of studies and studies that focused on respiratory illnesses, child growth, ocular infection, and mortality had mixed results.

Effects on widespread waterborne diseases (such as gastroenteritis, cholera, hepatitis, amoebiasis, and adenovirus) were scarcely analyzed in the studies included in this overview. Yet diarrhea, ultimately a symptom, reported in most cases as a dependent variable, may be related to the diagnosis of those diseases whose incidence is not typically measured as an outcome. Other outcomes, such as the adoption of healthy practices, take‐up rates, and behavior changes, did not show a definitive causal relationship to WASH interventions and require further empirical exploration.

Evaluations of combined interventions tended to consider behavioral outcomes. Such evaluations are important to understand if a combination of behaviors and the provision of physical infrastructure are driving impact results. However, at this point, 70 percent of behavior outcomes come from single‐ intervention evaluations of either hygiene or water quality trials (Figures A1 and A2).

# **4. Meta‐Analysis**

Different WASH interventions produce different results for the same outcome. For instance, water quality studies using a relatively common measurement of water treatment and health outcomes (e.g. incidence of diarrhea) varied widely in their estimates of the magnitude of outcome effects, depending on the approach or technology used. Thus, meta‐analysis is required to aggregate the point estimates and generalize the results. Ideally, all study designs are expected to have an adequate sample size; however, historically most WASH impact evaluations have been underpowered and have included small‐scale interventions. To counteract this problem, meta‐analysis is a powerful statistical tool that estimates the overall effects of different programs by pooling data that on their own would be too small to draw confident conclusions. The conclusions of a meta‐analysis are nonetheless based on the quality of the studies identified to estimate the pooled effect.<sup>9</sup>

# **4.1 Heterogeneity**

To assess heterogeneity, papers were categorized by geographical region and intervention subtheme and plotted against the standard error and precision of estimates. Figure A3 (see Appendix A) illustrates high

> 8 Neither chlorine treatment nor solar disinfection had significant impacts on diarrhea after a meta‐analysis adjusted for the nonblinding of the intervention, although an earlier systematic review and meta‐analysis of water quality interventions found household‐level treatment was more effective than source treatment (Hunter _et al_ . 2009).

> 9 The quality of randomized impact evaluations was evaluated and included with regard to the randomization, adequate assignment, and explanation of dropouts and withdrawals; this addresses the issues of both internal validity (minimization of bias) and external validity (ability to generalize results).

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