Conceptualising Natural and Quasi Experiments in Public Health

TL;DR

Natural and quasi experiments in public health are valuable for evaluating interventions that cannot be randomized, and aligning their design with the Target Trial Framework enhances clarity and causal inference.

Contribution

This paper conceptualizes natural experiments within public health, aligning them with the Target Trial Framework to improve their design, reporting, and causal inference.

Findings

Natural experiments combine features of experiments and observational studies.

Their validity depends on the plausibility of 'as-if' randomization.

Using the Target Trial Framework clarifies causal claims.

Abstract

Background: Natural or quasi experiments are appealing for public health research because they enable the evaluation of events or interventions that are difficult or impossible to manipulate experimentally, such as many policy and health system reforms. However, there remains ambiguity in the literature about their definition and how they differ from randomised controlled experiments and from other observational designs. Methods: We conceptualise natural experiments in in the context of public health evaluations, align the study design to the Target Trial Framework, and provide recommendation for improvement of their design and reporting. Results: Natural experiment studies combine features of experiments and non-experiments. They differ from RCTs in that exposure allocation is not controlled by researchers while they differ from other observational designs in that they evaluate the impact of event or exposure changes. As a result they are, in theory, less susceptible to bias than other observational study designs. Importantly, the strength of causal inferences relies on the plausibility that the exposure allocation can be considered "as-if randomised". The target trial framework provides a systematic basis for assessing the plausibility of such claims, and enables a structured method for assessing other design elements. Conclusions: Natural experiment studies should be considered a distinct study design rather than a set of tools for analyses of non-randomised interventions. Alignment of natural experiments to the Target Trial framework will clarify the strength of evidence underpinning claims about the effectiveness of public health interventions.