Risky business: making sense of estimates of risk

This article describes various indices of risk, which is the probability that a person will develop a specific outcome. The risk difference is the absolute difference in risks between 2 groups and can be used either to compare the outcome of 2 groups, one of which was exposed to some genetic or environmental factor, or to see how much of an effect a treatment may have. The reciprocal of the risk difference, the number needed to treat, expresses how many patients must receive the intervention in order for 1 person to derive some benefit. Attributable risk reflects the proportion of cases due to some putative cause and indicates the number of cases that can be averted if the cause were removed. Finally, the relative risk and odds ratio reflect the relative differences between groups in achieving some outcome, either good (a cure) or bad (development of a disorder).