2 - Explaining risk

Summary

Women may be concerned about the risks of any particular treatment and, in the case of menopausal women, HRT in particular. This has not been helped by media scares. This chapter focuses on how to explain risk and the risks in certain clinical situations.

Relative risk, absolute risk, attributable risk

The terms ‘relative risk’, ‘absolute risk’ and ‘attributable risk’ can be confusing. An understanding of the precise definitions is important to judge the actual magnitude of risks involved.

Relative risk (RR) is the risk of an event (or of developing a disease) relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus the control (non-exposed) group. It is often used to present clinical trial data, where it is used to compare the risk of developing a disease in people receiving a treatment compared with those not receiving treatment (or receiving a placebo). However, it does not take into account the actual frequency of the condition in the untreated group. For example, a relative risk of 2 could describe something that increased the risk of a disease from one in a million to two in a million or something that increased the risk of a disease from four people in ten to eight people in ten. Thus ‘absolute risk’ and ‘attributable risk’, which take into account the frequency of the condition, are better methods of presenting the data.