For instance, Freeman _et al_ . (2017) conducted a meta‐analysis to estimate pooled measures of the effects of different levels of sanitation services on infectious diseases and nutritional outcomes, primarily using studies with an experimental design. Stocks _et al_ . (2014) suggested sanitation services protect against diarrhea, active trachoma, some soil‐transmitted helminth (STH) infections, schistosomiasis, and height‐ for‐age reductions, and have no protective effect against other anthropometric outcomes. However, their meta‐analysis also highlighted the poor quality of the estimation methods used in the papers selected for study.

Garn _et al_ . (2017) conducted a meta‐analysis of 24 studies to examine the association between structural design characteristics of sanitation facilities and facility use, and found that most sanitation interventions had only a modest impact on increasing latrine coverage and use.<sup>6</sup> The authors combined the effects of sanitation interventions into a single outcome: latrine adoption and usage.

Studies that assessed health outcomes using RCTs<sup>7</sup> generally showed significant effects on the prevalence of diarrhea and waterborne diseases such as cholera (Taylor _et al_ . 2015). In contrast, differences in the incidence of diarrhea across groups were not significant in four quasi‐experimental studies. Evidence on other health‐related variables (e.g. child mortality, stunting, height, and weight) was scarce and limited to single studies, except for small‐scale studies evaluating combined interventions, which reported significant effects on mortality and stunting.

The evidence related to water quality was relatively solid, given that 75 percent of the trials that tested reductions in bacterial contamination reported strong effects. These studies were typically RCTs, implemented in rural zones, with in‐situ collection of water samples. Many of these were not explicitly labeled as water quality interventions, yet included a purification/filtration component as a complementary add‐on (frequently flocculants, chlorination, and ceramic filters) to strengthen the expected effect. In these combined interventions, the positive relationship was less evident and in 57 percent of the studies the effects were mixed.

Access to WASH was generally considered an intermediate outcome, since access is a direct result of water supply and sanitation interventions, especially those focused on improving infrastructure. Increases in access were well documented for water supply interventions and in seven out of eight studies effects were statistically verified. For on‐site water access, rigorous RCTs showed that outcome effects were half those of studies that relied on other methods to identify effects, which is consistent with what is reported by Ercumen, Gruber, and Colford (2014). In combined interventions the results were not strong, especially for outcomes difficult to measure (such as collecting time and distance).

WASH interventions are increasingly adding multiple components to achieve greater impacts on access, health, behavior change, and other socioeconomic indicators. Historically, WASH interventions have been skewed toward targeting particular health outcomes (such as reducing the incidence of diarrhea), negating other important outcomes. However, recent trends in WASH impact evaluations suggest more complex interventions can deliver results in multiple areas of development and well‐being.

> 6 A recent meta‐analysis of hygiene interventions found an average risk ratio for diarrhea of 0.60 for the promotion of handwashing with soap (95 percent CI: 0.53–0.68) and an average risk ratio of 0.76 for general hygiene education alone (95 percent CI: 0.67–0.86) (Hutton and Chase 2016; Freeman _et al_ . 2014). Other systematic reviews found a relative risk for respiratory infection of 0.84 (0.79–0.89) compared with no handwashing (Rabi and Curtis, 2006).

> 7 Including other research design methods such as quasi‐RCTs, non‐randomized controlled trials, controlled before‐and‐after studies, and cross‐sectional and uncontrolled before‐and‐after studies.

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