Broadly speaking, the ‘exposed to risk’ is the number of individuals within a specified population who are ‘at risk’ of a specified event occurring to them or in relation to them. For example, in a mortality analysis, the exposed to risk for males aged x would refer to the number of males who are alive at age x (who are ‘at risk’ of death). Statistically speaking the exposed-to-risk can be determined in one of two main ways: the ‘initial exposed to risk’ (in the absence of people joining during the year or leaving for reasons other than death) counts the population at the start of a period who will be at risk over the period whilst the ‘central exposed to risk’ counts the average number of people at risk over the period.

Exposed to risk need not be calculated in terms of numbers of people: other measures are possible – for example, amounts of pension are commonly used as the unit of measurement in mortality analyses.