| Disease Present (Cases) | Disease Absent (Controls) | Row Total |
|---|
| Exposed | a | b | a + b |
| Not Exposed | c | d | c + d |
| Column Total | a + c | b + d | Grand Total |
- Relative Risk (RR) compares PROBABILITY.
(a/a+c) is the probability of exposure in one group.- It’s a ratio of two probabilities.
- Use Relative Risk (RR) when:
- The study measures incidence (new cases over time).
- Study design is a Cohort Study or RCT. c
- Odds Ratio (OR) compares ODDS.
- Odds = (Number of times an event happens) / (Number of times it doesn’t).
(a/c) are the odds of exposure in the disease group (cases).(b/d) are the odds of exposure in the control group.- The OR is the ratio of these two odds.
- Use Odds Ratio (OR) when:
- The study design is a Case-Control Study.
- True incidence/prevalence cannot be determined because the investigator chooses the number of cases and controls.
- When the incidence is sufficiently low (rare disease), OR ≈ RR. c