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Table 1 Odds ratio, relative risk and risk difference

From: Bench-to-bedside review: Avoiding pitfalls in critical care meta-analysis – funnel plots, risk estimates, types of heterogeneity, baseline risk and the ecologic fallacy

 

Treatment

Placebo

 

Alive

a

b

(a + b) = e

Dead

c

d

(c + d) = f

 

(a + c) = g

(b + d) = h

i

  1. 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.