Computing absolute risk reduction or NNT from a risk ratio (RR)

To aid interpretation, review authors may wish to compute an absolute risk reduction or NNT from the results of a meta-analysis of risk 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 risk ratio is RR = 0.92, and an assumed control risk of ACR = 0.3 (300 per 1000) is assumed. Then the effect on risk is 24 fewer per 1000:


The NNT is 42: