Evidence-appraisal glossary
Absolute risk
Absolute risk is the actual probability of an event in a group, for example 2 in 100 people over a year. It contrasts with relative risk, which compares groups. Absolute figures show the real-world size of a risk or benefit, which relative percentages can exaggerate.
Also called: AR, absolute risk reduction, ARR.
Absolute risk is the plain chance that an event happens in a defined group over a defined time, expressed as a proportion such as 2 percent or 2 in 100. It matters because relative measures can distort perception: a treatment that cuts risk from 2 in 1,000 to 1 in 1,000 is a 50 percent relative reduction but only a 0.1 percentage-point absolute reduction, which sounds far less dramatic and is often more honest. When reading a study, look for the absolute numbers behind any relative claim; if only a percentage reduction is reported, ask what the baseline risk was, because a big relative change on a tiny baseline is a small change in real terms. The difference in absolute risk between two groups is the absolute risk reduction, and its inverse is the number needed to treat, another way to see real-world impact. For example, headlines touting a drug that halves risk mean little until you know whether the starting risk was 1 in 2 or 1 in 10,000.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.