22. Case-control studies

• Most frequently performed epidemiology studies
• A retrospective study – healthy and sick people are found, and their previous exposure to a factor is determined
• Aim: to study if an exposure (usually a risk factor) is associated with a disease
• Steps
  • Select patients with the disease = the cases
  • Select persons without the disease with similar baseline characteristics as the cases = the controls
  • Compare the exposure of risk factors between these groups
    • Find how many of the cases were exposed to the risk factor
    • Find how many of the cases were not exposed to the risk factor
    • Find how many of the controls were exposed to the risk factor
    • Find how many of the controls were not exposed to the risk factor
  • The odds ratio between the groups is determined
• Controls
  • Should be picked from the same source population as the cases
  • Must be at risk of the disease
  • Must have comparable characteristics as the cases
• Pros
  • Fast, cheap
  • Good for rare diseases
  • Can study multiple exposures
• Cons
  • Can’t determine incidence rate
  • Does not provide relative risk (RR)
  • Possibility of recall and selection bias
• Example
  • Richard Doll’s study which established the link between smoking and lung cancer