The ability to draw significant conclusions regarding Group 3 was limited by its small size. Nonetheless, there does not appear to be a relationship between study characteristics and outcomes here. Similarly, studies conducted in the same region were not correlated.

Studies within Group 4 were more homogenous than those of any other group, although most correlations were weak. This may be due to studies testing complementary interventions or impact evaluations being replicated in different locations. Experimental research methods (RCTs) and the implementing agency were both factors that contributed to the correlation between papers. The number of studies was significantly correlated to outcome effects and explained 78 percent of the variation, suggesting larger data sets produce more reliable results.

Finally, while certain aspects of papers within Group 5 were also significantly correlated, these papers had the widest variation in outcomes of all groups, making them especially suitable for aggregation. Two study characteristics were significantly and highly correlated with outcomes: i.e., study locale and implementing agency. Studies implemented by government agencies had a significant, negative impact on outcome effects, suggesting interventions implemented by NGOs produce more reliable results. Studies conducted in urban areas (64 percent of all studies) had a significant, negative impact on outcome effects even though all but one of those studies were implemented by NGOs.

# **6. Discussion**

Overall, there has been a large increase in WASH‐related impact evaluations over the past decade. Studies have been concentrated in the most underserved areas, while the East Asia and Pacific, Europe and Central Asia, and Middle East and North Africa regions remain underrepresented. Despite a large increase in studies evaluating combined WASH interventions, few evaluations target multicountry or multiregional interventions. Water quality interventions dominate impact evaluations, especially in Sub‐ Saharan Africa and Latin America and the Caribbean. This finding is not surprising, given that the more quantitative nature of water quality interventions makes them suitable for experimental methods. Although sanitation interventions were very common in India and behavior change interventions dominated in Bangladesh, these interventions were generally underrepresented in impact evaluations, making it difficult to assess regional effectiveness.

Despite a relatively high number of unique outcomes, more than half of all evaluations focused on diarrhea. This review corroborates the earlier findings of Hutton and Chase (2017) and Esteves Mills and Cumming (2016) that the positive effects on diarrhea reduction are well established and thoroughly documented, particularly for handwashing with soap and water quality trials. However, while experimental designs resulted in significant effects, quasi‐experimental designs did not produce the same results.<sup>12</sup> Evidence from the nonpooled studies on behavior change and other health‐related outcomes (e.g. child mortality, stunting, height, and weight) is scarce, and predominantly limited to single studies, although small‐scale studies evaluating combined interventions did report weak yet significant results for child mortality and stunting. Moreover, while a range of WASH interventions was frequently employed to control cholera outbreaks, few programs have been evaluated using rigorous impact evaluation

> 12 For instance, these studies reported that a recent meta‐analysis of five randomized controlled trials found a mean difference of 0.08 in height‐for‐age z‐scores of children under age five (95 percent CI: 0.00–0.16) for solar disinfection of water, provision of soap, and improvements in water quality (Dangour _et al_ . 2013).

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