- Also called follow-up studies, incidence studies, longitudinal studies, etc.
- Can be prospective or retrospective
- Prospective cohort study – healthy people with and without exposure to a factor is found and followed up until disease
- At the point of the study only the exposure, not the disease, has occurred
- Example: People who smoke and people who don’t smoke are chosen. They’re followed up until disease develops.
- Retrospective cohort study – a population with a disease is found. Historical data is used to separate the population into those who had the exposure and those who didn’t
- At the point of the study both the exposure and the disease has occurred
- Example: People with lung cancer are chosen. Their previous exposure to second-hand smoke is determined.
- Retrospective cohort studies are less important than prospective ones
- Assessment of exposure can be done in multiple ways
- By checking existing data, like medical records
- By giving questionnaires
- By measuring
- Prospective studies require an initially disease-free population
- The population must be screened for the disease
- Pros
- Good for rare exposures
- Give the strongest evidence for causation
- Give incidence rates, relative risk (RR) and odds radio (OR)
- Cons
- Not good for rare diseases
- Rxpensive
- Time-consuming
- Needs large sample size
- Comparison between retrospective cohort study and case control study
- In both, both the exposure and the disease have already occurred
- In a retrospective cohort study two similar populations are found, one exposed and one non-exposed. Both populations have the disease in question. The exposed and non-exposed group are compared, and the exposure’s effect on the outcome is determined.
- The RR and OR are calculated
- Only diseased people are examined
- In a case control study two similar populations are found, one with the disease and one without. Both populations are examined for exposure to the factor. The exposure’s effect on the outcome is determined.
- Only the OR can be calculated
- Both diseased (cases) and non-diseased (control) people are examined
- Examples
- British Doctors’ study (prospective cohort)