A worked example
Imagine a study where, over five years:
- 2 in 100 people without a treatment have a heart attack.
- 1 in 100 people with the treatment do.
The same result can be reported three ways:
- Relative risk reduction: the risk fell from 2 to 1 — a 50% reduction.
- Absolute risk reduction: 2% − 1% = 1 percentage point.
- Number needed to treat: 100 ÷ 1 = 100 people treated for five years for one of them to avoid a heart attack.
Why it matters
If the starting risk were 20 in 100 and fell to 10 in 100, the relative reduction would still be 50% — but the absolute reduction would be 10 points, and only 10 people would need treating for one to benefit. Relative figures sound the same; absolute figures show the real difference.
What to look for
- The starting risk, and the risk after — in the same group, over the same time.
- Both relative and absolute numbers.
- Side effects and costs, weighed against the benefit.
Sources
- Understanding Medical Research — MedlinePlus — U.S. National Library of Medicine. Checked 2026-09-26.
Editorial update: 2026-09-26.
Medical disclaimer: this is a source-linked educational summary, not individual medical advice, diagnosis, prescription or interpretation of personal results. For severe or rapidly worsening symptoms, call 115.
Share this guide
Published by
Antopeloni — plain-language health education from public sources, part of the ZIBADIS network founded by Masoud Moghaddam.
