Risk factors and prevention

Risk factors play a central part in prediction and prevention. A critical issue in any discussion of risk factors is to ensure that the term risk factor, and associated terms such as correlate and marker, are defined in a precise, consistent manner.1 The starting point is to understand that a correlate is a variable that is associated, either positively or negatively, with an outcome. The presence or absence of a correlate can be measured in each subject. The term subject need not be an individual but could be a family, classroom, school, or an entire community. The outcomes can be dimensional, but they are restricted in this discussion to binary ones. Binary outcomes are the most relevant in medical practice. The physician and patient are concerned about whether the patient has or does not have the illness or disorder. The correlate can be measured at the same time as the outcome and thus be a concomitant of it, or it can be measured after the outcome and be a consequence or result of it. A risk factor can be considered a type of correlate. It is associated with an increased probability of an outcome, usually an unpleasant one. It has, however, a major distinguishing characteristic from other correlates, which is it occurs before the outcome. This is 1 of the 2 defining characteristics of a risk factor. The measure of the risk factor is taken on each subject before the subject has the outcome of interest. The second defining characteristic of a risk factor is that it can be used to divide a population into high risk and low risk subgroups. The probability of the outcome must be shown to be greater in the high risk compared with the low risk group. Thus, the 2 defining characteristics of a …