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- 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
