improve water quality—on diarrheal morbidity. There is a clear need for a wider body of evidence on a broader range of WASH interventions and outcomes.

Therefore, the purpose of this study is twofold: (i) to collate new and existing evidence from impact evaluations of global WASH interventions into a single, publicly available repository; and (ii) to quantify the effects of a broad range of WASH interventions on an array of different outcomes, by performing a quantitative meta‐analysis of available evidence.

Conducting a systematic review and meta‐analysis of a single WASH‐related outcome is a time‐consuming endeavor. Expanding efforts to include the panoply of studies evaluating a diverse set of WASH outcomes around the globe is exponentially more laborious, as illustrated by the time lag between the commencement of this study and the publication of its results. Although a number of other, reputable impact evaluations and meta‐analyses evaluating the effects of WASH interventions on single outcomes have been published since this study began in 2013, this study remains the first of its kind to compare and contrast the effects of WASH interventions on diverse health and nonhealth outcomes through a combined meta‐analysis. As such, this study represents a critical, historical review of the effects of global WASH interventions that could be a valuable starting point for any future evaluation.

# **2. Selection of Impact Evaluations**

# **2.1 Inclusion Criteria**

To ensure the integrity of this review, only evidence‐based studies were included, that is, studies that rigorously measured impacts attributed to WASH interventions. Studies falling into this category include evaluations that identify causality between interventions and outcomes of interest by estimating the true effect (or impact) of an intervention on particular outcomes and applying counterfactual analyses (comparing intervention outcomes with what would have occurred in the absence of the intervention). Thus, all studies included in this review reported impact estimates along with their standard error, as is customary when conducting a meta‐analysis.

In terms of research methods, randomized controlled trials (RCTs) are considered the gold standard for establishing a causal attribution between interventions and outcomes. However, the choice to conduct an RCT may be limited by ethical considerations or study design, in which case experimental or quasi‐ experimental methods are instead applied. This review incorporates studies where RCTs, experimental design, and quasi‐experimental evaluation methods were applied.

In summary, only studies meeting the following criteria were included in this review: (i) they were completed before 2014 (ongoing evaluations or baseline findings not included); (ii) their results have been fully disclosed; (iii) they included quantitative evaluations with discernible comparison groups or an estimation of counterfactuals; and (iv) they included a clear, acceptable description of the impact identification strategy, as well as a description of the formal method used for comparison.

# **2.2 Search Methods**

As this review focuses exclusively on evidence‐based impact evaluations, an extensive yet targeted search of published and unpublished material was conducted. The primary source of published material was the Register of Impact Evaluation Published Studies (RIEPS) database, a comprehensive digital warehouse of impact evaluations with registered protocols in PubMed managed by the International Initiative for 3