This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Computing absolute risk reduction or NNT from an odds ratio (OR)

Review authors may wish to compute an absolute risk reduction or NNT from the results of a meta-analysis of odds ratios. In order to do this, an assumed control risk (ACR) is required. It will usually be appropriate to do this for a range of different ACRs. The computation proceeds as follows:


As an example, suppose the odds ratio is OR = 0.73, and a control risk of ACR = 0.3 is assumed. Then the effect on risk is 62 fewer per 1000:


The NNT is 17: