Free Relative Risk Calculator
Calculate the relative risk (risk ratio) from a 2x2 contingency table to compare event rates between two groups.
Relative Risk (RR)
2.0000
Risk Group 10.3000
Risk Group 20.1500
Risk Difference0.1500
NNT (Number Needed to Treat)6.67
Relative Risk (RR) vs Group 1 - Event (a)
How to Calculate Relative Risk
Formula
RR = [a / (a+b)] / [c / (c+d)]
Relative risk compares the probability of an event in the exposed group to the probability in the unexposed group. RR = 1 means no difference. RR > 1 means higher risk in the exposed group. RR < 1 means the exposure is protective. The Number Needed to Treat (NNT) = 1 /
Example Calculation
Group 1: 30 events out of 100. Group 2: 15 events out of 100.
- 01Risk Group 1 = 30 / (30+70) = 30/100 = 0.30
- 02Risk Group 2 = 15 / (15+85) = 15/100 = 0.15
- 03RR = 0.30 / 0.15 = 2.0
- 04Risk Difference = 0.30 - 0.15 = 0.15
- 05NNT = 1 / 0.15 ≈ 6.67
- 06Group 1 has twice the risk; for every ~7 people treated, one event is attributable to exposure.
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