We all know that the choice of the measure of effect (relative risk reduction [RRR], absolute risk reduction [ARR], or number needed to treat [NNT]) influences attitudes to a treatment. But how do we make this memorable for students? There are 2 types of “statistics”: those whose value depends on the underlying frequency of a disease and those statistics whose value does not depend on it (“invariant statistics”). Thus the RRR and its friends RBI (relative benefit increase) and RRI (relative risk …