based on example shown in Evidence-based Medicine (Sackett, Richardson,Rosenberg,Haynes) page 230, Churchill Livingstone, publishers.
| Group 1 | Group 2 |
| No of patients with event (r1) | No of patients with event(r2) |
| No of patients in group (n1) | No of patients in group (n2) |
| Proportion (%) of patients with event in group 1 (p1) | |
| Proportion (%) of patients with event in group 2 (p2) | |
| Absolute risk reduction (ARR) = p2-p1 (%) | |
| Standard Error (SE) of ARR; | |
| 95% Confidence Interval (CI) for ARR = ARR + 1.96*SE | - |
| Numbers Needed to Treat (NNT) = 100/ARR | |
| 95% CI for NNT = 100/CI for ARR | - |
| Relative Risk (RR) = p1/p2 | |
| 95% CI for RR | - |
| Relative Risk Reduction (RRR) = 1-RR (%) | |
| 95% CI for RRR = 1- CI for RR | - |
| Odds Ratio (OR) = r1(n2-r2)/r2(n1-r1) | |
| 95% CI for OR | - |