When reporting results for dichotomous outcomes (for example, alive or dead) of systematic reviews, the event rates are labelled “weighted event rates.” This weight reflects the relative contributions of individual studies to the overall risk difference, which is then used to compute the numbers needed to treat (NNTs).

The main factor in determining the weight for a trial is the number of events, which in turn depends on sample size, the event rate, and duration of follow up. Trials with a higher number of events get more weight. The number of events and total number of patients in each of the treatment arms are combined …