title: The contribution of natural experiments to the public health evidence base: four case studies in evidence synthesis
Creators: name: Hilton Boon, Michele L.
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Public health, natural experiments, regression discontinuity, systematic review, methodology, minimum legal drinking age, alcohol policy, policy evaluation, breast cancer, environment, endocrine disruptors, evidence synthesis, meta-review.

description: Background: Natural experiments and related study designs such as regression discontinuity (RD) are of increasing interest to researchers and decision makers because of their potential to address confounding and selection effects better than other observational study designs, with potentially greater applicability than controlled experiments. Research methods in health have been relatively slow to incorporate natural experiments compared to other fields such as economics and political science, but interest in these methods is growing rapidly. Objectives: This thesis aimed to (1) investigate the contribution of natural experimental designs to public health research, specifically the evaluation of public health interventions and environmental causes of disease and (2) explore how systematic review methods might be applied to make better use of natural experiments to inform public health and policy. Methods: The thesis comprises four case studies, including a systematic review of RD studies of health outcomes, a systematic review of RD studies of minimum legal drinking age (MLDA) legislation, development of a critical appraisal tool for RD studies, and a meta-review of endocrine-disrupting chemicals (EDCs) and breast cancer risk. Review protocols were registered in the PROSPERO database. Results: The first systematic review identified 181 RD studies of health outcomes which spanned a wide range of public health and policy questions, showing that this natural experimental design has been more widely applied than previously appreciated. Thematic analysis of the forcing variables and threshold rules used in these studies will aid in future applications of the design. The MLDA review of 17 econometric analyses identified challenges in the synthesis of natural experimental studies. The review identified evidence that MLDA has a causal effect on mortality and on alcohol-related hospital admissions. A ten-item checklist specific to the methodological requirements of RD designs was developed based on standards for RD produced by the What Works Clearinghouse; only 5% of the 181 studies met all ten criteria. The meta-review included 15 systematic reviews of EDCs and breast cancer risk; no primary studies in the review were identified as natural experiments. Conclusions: Natural experiments have the potential to support stronger causal inference through designs that address selection effects and confounding. For these designs to be translated into better evidence to inform decision-making, systematic reviews need to be able to identify and represent in detail the differences among non-randomised study designs. To do this requires further development of systematic review methods in order to synthesise results from econometric models and assess the quality of natural experimental studies.