Relative Risk vs Absolute Risk
A fifty per cent reduction can mean a great deal or almost nothing. The difference is which number was reported.
Last reviewed September 10, 2026
Relative risk describes a proportional change between groups. Absolute risk describes how many people in a hundred were actually affected. Headlines prefer the first; decisions require the second.
A worked illustration
If an event occurs in two people out of a thousand and an intervention reduces that to one, the relative reduction is fifty per cent and the absolute reduction is one tenth of one percentage point.
Both statements are true. Only one of them conveys the scale of the benefit.
Numbers needed to treat
The number needed to treat converts absolute difference into people: how many must receive an intervention for one to benefit. It is the most honest single figure in clinical research and one of the least quoted.
When a claim gives you only a percentage, look for the event rates in each arm before forming a view.