The Number Needed to Treat (NNT) is most directly derived from which measure?

• A. Absolute risk reduction
• B. Relative risk reduction
• C. Hazard ratio
• D. Odds ratio

Answer: (A)

Number Needed to Treat is grounded in the actual difference in event risk between groups, the absolute risk reduction. ARR is calculated as the risk in the control group minus the risk in the treated group (ARR = CER − EER). NNT is then the reciprocal of this difference: NNT = 1 / ARR.

For example, if 10% of untreated patients have the adverse event and 6% of treated patients have it, ARR = 0.04, so NNT = 1 / 0.04 = 25. This means you’d need to treat 25 people to prevent one event over the specified time.

Relative measures like relative risk reduction, hazard ratio, or odds ratio describe proportional changes or time-to-event relationships and don’t directly give the absolute difference in risk. Because baseline risk varies, the same relative effect can yield different NNTs, and NNT depends on the time horizon as well. That’s why NNT is tied to absolute risk reduction.