How is the number needed to treat (NNT) calculated?

The number needed to treat (NNT) is a critical measure in clinical research used to determine how many patients need to be treated with a specific therapy in order to prevent one adverse outcome (such as a disease, complication, or death). The correct calculation is based on the absolute risk reduction (ARR), which quantifies the difference in event rates between the control group and the treatment group.

To calculate NNT, the formula is NNT = 1 / ARR. Here, ARR is derived by subtracting the event rate in the treatment group from the event rate in the control group. By using this formula, you effectively translate the clinical benefit of a treatment into a tangible figure—indicating how many individuals must be treated to prevent one additional bad outcome. A lower NNT indicates a more effective treatment, as fewer people need to be treated to observe a benefit.

Understanding how to derive NNT is crucial for healthcare providers making informed decisions about treatment options, as it provides context not only for efficacy but also for the broader impact of treatments across populations.