Re: Estimating the Effect of School Water, Sanitation, and Hygiene Improvements on Pupil Health Outcomes

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In their excellent re-analysis of a cluster-randomized trial that delivered combinations of water, sanitation, and handwashing (WASH) improvements to schools in rural Kenya, Garn and colleagues1 showed how imperfect compliance likely attenuated some intention-to-treat estimates by comparing them with complier average causal effect estimates—the effect of the intervention among the subgroup of schools that complied with their intervention assignment. Because the subgroup of compliers cannot be observed in a parallel arm trial,2 Garn and colleagues1 used an instrumental variables (IV) technique to estimate complier average causal effects, and should be commended for extending the original trial’s inference.
The article illustrates two more general issues related to compliance definitions in community intervention trials. First, community trials that wish to use IV techniques to estimate complier average causal effects for infectious outcomes should define compliance at the level of randomization—individual-level definitions are generally problematic (Figure). Second, the definition of compliance for a multifaceted intervention is centrally important for complier average causal effects, “per-protocol” and “as treated” parameters,3 yet defining compliance is rarely straightforward. A vaccine trial delivers treatment at specific ages or time points, which can be measured and defined unambiguously. This is not the case for many community level interventions—instead, they often have multiple components that involve behavior change, which can vary during follow-up. Garn and colleagues1 used a pragmatic compliance definition based on a count of water, sanitation, and handwashing components present on a single visit during follow-up. Their definition weighted each intervention component equally and included no information about behavior change over the 18-month follow-up. Yet, each intervention component could be more or less important depending on the dominant pathogen mix, and actual use of the hardware should be important to the intervention’s efficacy. This example illustrates how compliance definitions can be complex, subject to data availability, and ultimately debatable. The clinical trials literature has developed compliance definitions based on prespecified treatment regimens over time,3 and a similar approach may be helpful for community level interventions with time-varying compliance. More complex compliance definitions may reduce complier misclassification but may also open more room for debate. Because multiple compliance definitions may be reasonable in complex community interventions, and the choice has direct bearing on the magnitude and interpretation of complier average causal effects or other measures that rely on identifying compliers,3 it would be prudent for trials to prespecify, justify, and fully report results for each definition.
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