Introduction

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

In this three paper series we will provide an approach to the critical appraisal of cohort studies. This article describes the role and design of cohort studies and explains how selection bias can confound the relation between the intervention and the outcome. The second article will outline strategies for identification and assessment of the potential for confounding, and the third article describes statistical techniques that can be used to deal with confounding. Each paper defines a set of questions that, taken together, can provide readers with a systematic approach to critically assessing evidence from cohort studies.

Randomised trial or cohort study?

Cohort studies are similar to randomised controlled trials in that they compare outcomes in groups that did and did not receive an intervention. The main difference is that allocation of individuals is not by chance. 1 gives some important similarities and differences between the two types of study. Because they are expensive and recruiting …