Absolute risk

Treatment decisions in the prevention and management of coronary heart disease (CHD) often require knowledge about the level of risk. Risk is defined as the probability of encountering a particular event. Information about risk is obtained from cohort studies or other longitudinal studies such as randomised controlled trials. In these studies, risk is simply the incidence of the event in a particular group. For example, the risk (or probability) of death associated with coronary angiography is 0.1% (or 1 in 1000).1 This is termed the absolute risk.

In order to inform clinical decisions, it is often useful to compare the absolute risk in two or more groups having different exposures or different treatments (including treatment versus no treatment). The two main measures we use when comparing risk are attributable risk (risk difference) and relative risk (risk ratio).

Attributable risk

Attributable risk measures the excess risk accounted for by exposure to a particular factor.2 This is simply the difference between the absolute risks in the two groups. The term attributable risk is most commonly used in epidemiological studies. In the clinical environment, when comparing a particular treatment with placebo for example, the excess risk associated with treatment (attributable risk) may well be negative, if the treatment is beneficial. This is therefore termed an absolute risk reduction. Absolute risk reduction …