| Treatment | Placebo | |
---|
Alive | a | b | (a + b) = e |
Dead | c | d | (c + d) = f |
| (a + c) = g | (b + d) = h | i |
- Risk difference = risk of death with treatment – risk of death with placebo = (c/g) – (d/g). Risk ratio = risk of death with treatment/risk of death with placebo = (c/f)/(b/e). Odds ratio = odds of death with treatment/odds of death with placebo = (c/a)/(d/b). The odds ratio approximates the relative risk when the event rate (here, death) is uncommon; however, when the event rate is common the odds ratio will overestimate the relative risk.