Non-clinical sciences/Stats & evidence · Effect measures

NNT & NNH

What it means

NNT = 1/ARR — patients treated to prevent one additional bad outcome (lower = more effective). NNH = 1/ARI — patients exposed to cause one additional harm. Always quote the time-frame and comparator.

Worked example

If a drug cuts absolute event rate from 10%→5% (ARR 5% = 0.05), NNT = 1/0.05 = 20.

In the exam

A trial reduces the absolute event rate by 5 percentage points; the reciprocal of that gives the number of patients you must treat to prevent one event.

What settles it

NNT comes from the ABSOLUTE risk reduction, not the relative — a big RRR on a rare outcome can still give a huge (poor) NNT.

Classically confused with

Relative risk & odds ratio

Source: StatPearls