Reader's guide to critical appraisal of cohort studies: 1. Role and design

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Abstract
Valid evidence on the benefits and risks of healthcare interventions is essential to rational decision making. Randomised controlled trials are considered the best method for providing evidence on efficacy. However, they face important ethical and logistical constraints and have been criticised for focusing on highly selected populations and outcomes.1 Some of these problems can be overcome by cohort studies. Cohort studies can be thought of as natural experiments in which outcomes are measured in real world rather than experimental settings. They can evaluate large groups of diverse individuals, follow them for long periods, and provide information on a range of outcomes, including rare adverse events. However, the promise of cohort studies as a useful source of evidence needs to be balanced against concerns about the validity of that evidence.3