risikoratio

Risikoratio, in English commonly called relative risk or risk ratio (RR), is a statistical measure used to compare the probability of a specified outcome occurring between two groups. It is widely employed in epidemiology, public health, and clinical research to express how much more (or less) likely an event is in one group compared with another.

The risk ratio is defined as the ratio of the risk in the exposed group to the risk in the unexposed group. If a study reports a 2x2 table, where A is the number with the event among the exposed, B is the number without the event among the exposed, C is the number with the event among the unexposed, and D is the number without the event among the unexposed, then RR = [A/(A+B)] / [C/(C+D)]. A risk in a group is the incidence of the outcome during the study period.

Interpretation follows intuitive rules: RR = 1 indicates no difference in risk between groups; RR > 1 indicates higher risk in the exposed group; RR < 1 indicates lower risk in the exposed group. The magnitude reflects relative, not absolute, risk. For example, RR = 3 means the event is three times as likely in the exposed group as in the unexposed group.

Risikoratio is typically derived from cohort studies and randomized trials where incidence can be measured over time. It is less directly estimable in case-control studies, where the odds ratio is often used as an approximation. Limitations include confounding, bias, and the dependence on follow-up duration; RR should be interpreted alongside absolute risk differences and confidence intervals.