A cohort study assembles a group of patients and follows them forward over time

One of the best know examples is the Doll and Hill studies on a cohort of British Doctors smoking habits which showed the link between smoking and lung cancer.

Cohort studies can also be used to assess prognosis by following patients after they develop the disease in question to see how they do regarding mortality, disease progression, and other important outcomes.


  • ethically safe
  • subjects can be matched
  • can monitor changes over time, establish timing and directionality of events
  • can investigate effects of rare exposure
  • can monitor more than one outcome
  • eligibility criteria and outcome assessments can be standardised
  • administratively easier and cheaper than RCT.


  • controls may be difficult to identify
  • exposure may be linked to a hidden confounder
  • blinding is difficult
  • randomisation not present
  • for rare disease, large sample sizes or long follow-up necessary which increases cost
  • follow-up can be difficult.



Other Resources

  • On-line lecture – Part of Principles of Epidemiology Series from University of Pittsburgh
  • Cohort studies– from the EBD series on study design by Kate Levin