What is the formula for calculating the number needed to harm (NNH)?

The formula for calculating the number needed to harm (NNH) is indeed based on the differences in event rates between a treatment group and a placebo or control group. The correct formulation states that NNH equals 1 divided by the difference in event rates of the harm occurring in the treated group versus the rate in the control group.

Specifically, NNH is calculated as 1 divided by the event rate in the treatment group minus the event rate in the placebo group. This reflects the number of patients who would need to be treated with a particular intervention for one additional patient to experience an adverse event compared to those who do not receive the treatment.

This is crucial for understanding the potential risks associated with treatments in clinical practice. The greater the difference in event rates, the smaller the NNH, meaning that fewer patients need to be harmed to observe one adverse effect.

Understanding this formula helps clinicians assess the safety profile of treatments and weigh them against their benefits. In contrast, other options such as the calculations involving absolute risk reduction (ARR) or attributable risk decrease focus more on benefit rather than harm, which is outside the scope of determining NNH. The correct application of these formulas is vital in evidence-based medicine, ensuring that patient safety is prioritized alongside