,NNTs can be calculated for single studies as follows. Note that this approach, although applicable, should only very rarely be used for the results of a meta-analysis of risk differences, because meta-analyses should usually be undertaken using a relative measure of effect (RR or OR).
A NNT may be computed from a risk difference as
,
where the vertical bars (‘absolute value of’) in the denominator indicate that any minus sign should be ignored. It is convention to round the NNT up to the nearest whole number. For example, if the risk difference is –0.12 the NNT is 9; if the risk difference is –0.22 the NNT is 5.