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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 is increasingly used when reporting clinical trials. The absolute risk reduction enables calculation of the "number needed to treat" (NNT). The NNT is simply the inverse of the absolute risk reduction. 3 The NNT represents the number of patients that need to be treated to prevent one adverse event.
Relative risk
Relative risk is the ratio of two absolute risks. It measures the strength of effect of an exposure (or treatment) on risk. 2 A beneficial treatment will result in a relative risk of less than 1; this can then be subtracted from 1 to give the relative risk reduction. A harmful treatment, or other exposure, will give a relative risk of more than 1.
In the British doctors study, over 40 years of follow up, the annual mortality rate from CHD was 572 per 100 000 in non-smokers, and 892 per 100 000 in...