Absolute vs relative riskRelative risk reduction · Absolute risk reduction

Relative risk expresses a change as a proportion of the starting risk: a fall from 2 in 1,000 to 1 in 1,000 is a 50% relative reduction. Absolute risk expresses the same change as the difference itself: 1 fewer case per 1,000 people. Both describe the identical finding, and they read completely differently.

It matters because headlines and marketing almost always quote the relative figure, since it is the larger-sounding number. Without the baseline risk, a relative change cannot tell you whether an intervention is worth doing.

In practice, look for the absolute numbers, or the number needed to treat — how many people must do the thing for one to benefit. If a claim only ever appears as a percentage change, that is usually a choice rather than an oversight.

Sources

• Centre for Evidence-Based Medicine, University of Oxford: explanation of the 2011 OCEBM Levels of Evidence