Number Needed to Treat

Number Needed to Treat tells you how many people must receive an intervention for one extra person to have a good outcome compared with not receiving it. It comes from the absolute risk reduction because the benefit is the fraction of patients who are helped beyond what would have happened anyway; taking 1 divided by that fraction converts a per-patient difference into a "people needed" count. A smaller ARR means a rarer benefit, so more patients are needed to see one additional success.

Use this when comparing a treatment or prevention strategy against a control in clinical trials, screening studies, or public health programs. It is especially useful for judging how practical or impactful an intervention is in real-world decision-making.

• Using the treatment rate or control rate directly instead of the absolute risk reduction
• Forgetting that ARR must be written as a decimal, not a percent
• Rounding too early and getting an inaccurate NNT