What does the number needed to treat (NNT) represent in a cohort study?

• A. The number of events prevented per year.
• B. The number of individuals who must be exposed to a protective factor to prevent one adverse outcome.
• C. The number of participants required to observe one outcome.
• D. The inverse of the risk ratio.

Answer: (B)

NNT tells you how many people need to receive a protective exposure to prevent one adverse outcome over a defined time period. It comes from the absolute risk reduction between exposed and unexposed groups: NNT = 1 / (risk in unexposed − risk in exposed). So if the protective factor lowers risk from 10% to 5% over the study period, the ARR is 0.05 and the NNT is 20. That means 20 people must be exposed to prevent one event within that time frame.

This differs from counting events per year or from simply taking the inverse of a risk ratio (which is 1/RR, not 1/ARR). In observational cohort data, be aware that confounding can affect the estimate, so the NNT should be interpreted in light of potential biases.