Primary Measure of Association in Cohort Studies
Key Point
In cohort studies, the relative risk (RR) is the primary measure of association because cohort studies follow exposed and unexposed groups forward in time to observe the development of disease.
Why Relative Risk?
Cohort studies are prospective (or can be retrospective but still follow the cohort structure). Because the investigator:
- 1.
Identifies exposed and unexposed individuals at baseline
- 2.
Follows them forward to measure disease incidence in each group
- 3.
Can directly calculate the incidence in exposed and incidence in unexposed
The relative risk is calculated as:
RR=Incidence in unexposedIncidence in exposed=c/(c+d)a/(a+b)
where the 2×2 table has disease (yes/no) as columns and exposure (yes/no) as rows.
Comparison with Case-Control Studies
| Feature | Cohort Study | Case-Control Study |
|---|
| Primary measure | Relative Risk (RR) | Odds Ratio (OR) |
| Direction | Exposure → Disease | Disease ← Exposure |
| Incidence calculable? | Yes | No |
| Why OR in case-control? | Cannot calculate true incidence; disease prevalence in sample is artificially fixed by design | — |
High-YieldNEET PG
RR directly answers "How many times more likely is disease in the exposed group?" This is only possible in cohort studies where you follow groups forward and measure true incidence.
Clinical Pearl
An RR of 2.5 means exposed individuals are 2.5 times more likely to develop the disease than unexposed individuals—a direct, intuitive interpretation.
