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Direct computation of an absolute risk reduction (ARR) or a number needed to treat (NNT) depends on the summary statistic (odds ratio, risk ratio or risk differences) available from the study or meta-analysis. When expressing results of meta-analyses, authors should use, in the computations, whatever statistic they determined to be the most appropriate summary for pooling (see Chapter 9, Section 9.4.4.4). Here we present calculations to obtain ARR as a reduction in the number of participants per 1000. For example, a risk difference of –0.133 corresponds to 133 *fewer* participants with the event per 1000.

ARRs and NNTs should not be computed from the aggregated total numbers of participants and events across the trials. This approach ignores the randomization within studies, and may produce seriously misleading results if there is unbalanced randomization in any of the studies.

When computing NNTs, the values obtained are by convention always rounded up to the next whole number.