Natural experiments: an underused tool for public health?
Section snippets
Non-comparability of intervention and control groups
As with any observational study, there are many potential sources of bias. Chief among these is the likelihood that the control and intervention groups are dissimilar at baseline in ways that are related to health outcomes. In randomized controlled trials, both known and unknown factors are distributed at random, and therefore any difference in outcome can only be due to the intervention.14 By contrast, in natural experiments, health outcomes may differ amongst groups because of baseline
Natural experiments: the benefits
One potential benefit of natural experiments is their external validity, as they provide assessments of effectiveness rather than efficacy. However, this may be compromised by low internal validity.23 Their generalizability needs consideration, as the intervention may be so locally specific that study results cannot be generalized to other areas, even within the same country. This may necessitate multisite evaluations to explore effects in different settings. For example, the SHARP study is
Conclusion
Natural experiments are common in real life but are not in common use in public health research. Randomized controlled trials do represent ‘best available evidence’, but in many public health settings, they unfortunately represent best unavailable evidence because they can be problematic to achieve, or simply because of the unwillingness of governments to consider implementing interventions in a manner that would facilitate robust outcome assessment.2 A commitment to the use of natural
Acknowledgements
The work on the hypermarket study was undertaken by MP, SC, LS and AF, who received funding from the Department of Health. The views expressed in the publication are those of the authors and not necessarily those of the Department of Health. The SHARP study (conducted by MP, CH and AK) is funded by the Chief Scientist Office of the Scottish Executive Department of Health, and Communities Scotland. Mark Petticrew is funded by the Chief Scientist Office of the Scottish Executive Department of
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