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Research Methods in Pharmacology Quiz
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Which of the following statements about the number needed to treat (NNT) is correct?
- A It is calculated as 1 divided by the relative risk reduction. You picked ✕
- B It is the number of patients who must be treated for one additional patient to be harmed. You picked ✕
- C It is calculated by subtracting the event rate in the treatment group from the event rate in the control group. You picked ✕
- D It is calculated as 1 divided by the absolute risk reduction, expressed as a decimal. You picked ✓
- E It is calculated as the absolute risk reduction divided by the event rate in the control group. You picked ✕
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The answer is D: It is calculated as 1 divided by the absolute risk reduction, expressed as a decimal.
Explanation
Why D is correct
See Number Needed to Treat. Option D is correct. The number needed to treat (NNT) is the number of people who must be given a treatment, instead of the comparator, for one additional person to benefit over a stated follow-up period. It is calculated as 1 divided by the absolute risk reduction (ARR). The absolute risk reduction is the event rate in the comparison group minus the event rate in the treated group. For example, if 20 percent of the comparison group have a stroke compared with 10 percent of the treated group, the ARR is 0.20 minus 0.10 = 0.10, so the NNT is 1 divided by 0.10 = 10. Treating 10 people, rather than giving the comparator, is expected to prevent one additional stroke. This does not mean that exactly one in every 10 people benefits; it is an average. The NNT conveys both statistical and clinical importance, and a smaller NNT means a more effective treatment. The matching measure for harm is the number needed to harm (NNH).
- Formula: the number needed to treat is 1 divided by the absolute risk reduction.
- Worked example: 20 percent against 10 percent gives an absolute risk reduction of 0.10, so the number needed to treat is 10.
- Direction: a smaller number needed to treat means a more effective treatment.
Why the other options are incorrect
- A. It is calculated as 1 divided by the relative risk reduction.: NNT uses the absolute difference in risk and not the relative reduction. Dividing 1 by a relative reduction gives a misleadingly small answer when the baseline risk is low: halving a risk of 2 in 1000 gives 1 divided by 0.5, which is 2, yet only 1 event in every 1000 people treated is prevented, so the true NNT is 1000. Your answer
- B. It is the number of patients who must be treated for one additional patient to be harmed.: This is the number needed to harm, not the number needed to treat. The NNT concerns one additional beneficial outcome, whereas harm is captured by the number needed to harm; using one for the other reverses the intended measure. Your answer
- C. It is calculated by subtracting the event rate in the treatment group from the event rate in the control group.: This calculation gives the absolute risk reduction itself, not the number needed to treat. The NNT is one divided by the absolute risk reduction, so subtracting the event rates gives the very number that then has to be inverted. Your answer
- E. It is calculated as the absolute risk reduction divided by the event rate in the control group.: This formula gives the relative risk reduction, not the number needed to treat. The NNT is the reciprocal of the absolute risk reduction, so dividing the absolute risk reduction by the control event rate leaves a relative measure rather than the number of patients who need treating. Your answer
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