Evidence-appraisal glossary
Number needed to treat
Number needed to treat (NNT) is the number of people who must receive a treatment for one additional person to benefit, over a given time. It is the reciprocal of the absolute risk reduction: an absolute reduction of 5 percentage points gives an NNT of 20.
Also called: NNT.
Number needed to treat turns a study's absolute benefit into a single tangible count: how many people you would have to treat to prevent one extra bad outcome. You get it by dividing 1 by the absolute risk reduction expressed as a proportion, so a reduction from 10 percent to 5 percent, an absolute drop of 0.05, yields an NNT of 20. Lower numbers mean more effective treatments; an NNT of 5 is stronger than an NNT of 100. When reading a study, check the time horizon, because an NNT of 50 over one year is very different from 50 over ten years, and confirm the outcome being counted is one that matters, not a surrogate marker. NNT is undefined or misleading when a treatment shows no real benefit. It always travels with a companion, number needed to harm, and honest appraisal weighs the two together rather than citing benefit alone.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.