number needed to treat (NNT)

The number of subjects treated per subject with a favourable outcome.

Note: It is the inverse of absolute risk reduction (1 ÷ absolute risk reduction). Thus, if the results of a study indicate that the probability of death in a control group is 25% and the probability of death in a treatment group is 10%, the number needed to treat would be 1.0 ÷ (0.25 - 0.10) = 6.7, therefore 7 subjects.

Related concepts: risk difference, absolute risk reduction, relative risk reduction and odds ratio