The number of patients that need to be treated to prevent the health outcome of interest.

The number needed to treat is used to provide a more intuitive picture of the efficacy or protective value of an intervention. It can be calculated using the formula NNT = 100/ARR, where ARR is the absolute risk reduction (the difference between event rates in control and intervention groups as a percentage).

NNT can provide a more understandable picture of the benefit of an intervention than the commonly used relative risk reduction (RRR, the difference between events rates divided by the rate in the control group).

For example, if an intervention reduces event rates from 20% to 10%, the RRR is 50%, the ARR is 10%, and the NNT is 10. However, if an intervention reduces event rates from 4% to 2%, the RRR is still 50%, but the ARR is 2%, and the NNT is 50.