This is an archived version. For the current version, please go to training.cochrane.org/handbook/current.

Because risk ratios are easier to interpret than odds ratios, but odds ratios have favourable mathematical properties, a review author may decide to undertake a meta-analysis based on odds ratios, but to express the result as a summary risk ratio (or relative risk reduction). This requires an assumed control risk (ACR). Then

It will often be reasonable to perform this transformation using the median control group risk from the studies in the meta-analysis.