The number needed to treat: problemsdescribing non-significant results

If 5% of people receiving cognitive behavioural therapy relapsed after treatment and 25% of people receiving psychotherapy relapsed, we could express this as an absolute risk reduction: ARR = absolute difference in event rates (25-5%) = 20% (interpretation: cognitive behavioural therapy reduces the absolute risk of relapse by 20% compared with psychotherapy) Alternatively, we could describe the result as a number needed to treat: NNT = 1/ARR (1/20%) = 5 (interpretation: treat five people with cognitive behavioural therapy rather than psychotherapy to prevent one relapse) The NNT allows us to dispense with percentage risks and helps describe effects in terms of real people. To obtain the corresponding NNTs, we invert each percentage. [...]we obtain NNT 20, 95% CI -20 to +10.