Glossary

Absolute and relative risk

Relative risk is the risk ratio of two groups; absolute risk is how many people have the event. One relative cut gives different absolute gaps by baseline risk.

Written by Metehan Yılmaz
In this guide

Absolute risk is how many people actually experience an event over a given period (for example, 4 in 1,000 people). Relative risk (risk ratio, RR) is the ratio of the risk in two groups. The Cochrane Handbook explains that the risk ratio and odds ratio are relative measures, while the risk difference and the number needed to treat (NNT) are among the absolute measures; it states that even when relative effects are similar across subgroups, absolute effects will vary with baseline risk. [2]

Why are they read separately?

Gigerenzer and colleagues illustrate the difference with a mammography example: the statement that screening reduces the risk of death from breast cancer by 25% actually means that 1 in 1,000 women dies less from the disease. [1] A hypothetical example shows the same calculation: if an event falls from 4 to 3 in 1,000 people, the relative reduction is 25% and the absolute reduction is 1 in 1,000 (0.1 percentage points); in this case the NNT is 1,000.

What is NNT?

As defined by the Cochrane Handbook, NNT is the expected number of people who need to receive the experimental intervention rather than the comparator for one additional event to occur or be prevented; it is given for a specific time frame. [2]

When reading

If a headline only says “X% reduction,” it is unclear how common the event was at baseline. The confidence interval is also read separately (Confidence interval). General reading order: How to read a study?.

Sources

  1. Gigerenzer G et al. — Helping doctors and patients make sense of health statistics (Psychol Sci Public Interest) (2007)
  2. Schünemann HJ et al. — Cochrane Handbook for Systematic Reviews of Interventions, Bölüm 15: Interpreting results and drawing conclusions (sürüm 6.5) (2024)

Updated

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