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The number needed to treat: problemsdescribing non-significant results
  1. Vivek Muthu, Dr
  1. Medical Director

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The number needed to treat (NNT) and its counterpart, the number needed to harm (NNH), are commonly used measures of effect in studies of mental health treatments. The NNT is an absolute measure, which conveys a more direct and intuitive impression of the effect on an individual than the absolute risk reduction (ARR).

To illustrate this, consider an imaginary trial comparing the effects of cognitive behavioural therapy versus interpersonal psychotherapy for people with bulimia. 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. Confidence limits can also be provided for the NNT, which, loosely interpreted give us an idea of the range of effect we might expect most of the time. When the confidence interval includes a situation where intervention has no effect, the result is …

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