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Cohort studies

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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In cohort studies groups of people are studied over a period of time, with certain characteristics determined at the start of the study. The cohort study is a longitudinal study. The study follows forwards over a fixed period of time two groups of people who have different exposures to a particular agent e.g. HRT and non-HRT takers, but are otherwise matched. The incidence of disease is then observed in each of the groups.

The objective of cohort studies is to test aetiological hypotheses and estimate chronic health effects.

A cohort study is a prospective study in contrast to a case control study which is a retrospective study.

In a cohort study the groups are compared with respect to disease e.g.

  • HRT users -----------> incidence of breast cancer
  • non-HRT users -----------> incidence of breast cancer

A BMJ review (1) suggested some characteristics of cohort studies:

  • populations studied
    • diverse populations of patients who are observed in a range of settings
  • allocation to the intervention
    • based on decisions made by providers or patients
  • outcomes
    • can be defined after the intervention and can include rare or unexpected events
  • follow-up
    • many cohort studies rely on existing experience (retrospective studies) and can provide an opportunity for long follow-up
  • analysis
    • sophisticated multivariate techniques may be required to deal with confounding

Case-control study:

  • Establish proportion with <--------- cases of DVT/PE past exposure to HRT in menopausal women
  • Establish proportion with <--------- no DVT/PE in past exposure menopausal women


  1. Rochon PA et al.Reader's guide to critical appraisal of cohort studies: 1. Role and design. BMJ 2005;330:895-7.

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