Intended for healthcare professionals

Education And Debate

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

BMJ 2005; 330 doi: (Published 14 April 2005) Cite this as: BMJ 2005;330:895

This article has a correction. Please see:

  1. Paula A Rochon, senior scientist1,
  2. Jerry H Gurwitz, executive director2,
  3. Kathy Sykora, senior biostatistician3,
  4. Muhammad Mamdani, senior scientist3,
  5. David L Streiner, professor4,
  6. Susan Garfinkel, research coordinator3,
  7. Sharon-Lise T Normand, professor of health care policy (biostatistics5,
  8. M Geoffrey (, chair in health management strategies6
  1. 1 Kunin-Lunenfeld Applied Research Unit, Baycrest Centre for Geriatric Care, Toronto, ON, Canada
  2. 2Meyers Primary Care Institute, Worcester, MA 01605, USA
  3. 3Institute for Clinical Evaluative Sciences, Toronto, ON, Canada
  4. 4Department of Psychiatry, University of Toronto, Toronto, ON, Canada
  5. 5Department of Health Care Policy, Harvard Medical School, Boston, USA
  6. 6Department of Health Policy, Management, and Evaluation, Faculty of Medicine, University of Toronto, Toronto, ON, Canada
  1. Correspondence to: G M Anderson, Institute for Clinical Evaluative Sciences, 2075 Bayview Avenue, Toronto, ON M4N 3M5, Canada
  • Accepted 18 February 2005

Cohort studies can provide valuable information unavailable from randomised trials, but readers need to be alert to possible flaws]


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 …

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