On the interpretation of risk and rate advancement periods

Risk and rate advancement periods (RAP) measure the impact of an exposure on the relation of age to disease. Specifically, they quantify the time by which the risk or rate of a disease is advanced among exposed subjects conditional on disease-free survival to a certain baseline age. The fact that these measures incorporate timing of disease occurrence makes them appealing for risk communication, and their use has increased over recent years. Unfortunately, their misinterpretations have also increased. In particular, RAP is often misinterpreted as the difference in mean survival time, when in fact it is a distinct concept and a profoundly different quantity. Other misinterpretations include interpreting RAP as the time by which a survival curve is shifted between exposed and unexposed, and equating RAP to a simple function of relative risks. In this paper we review RAP and show why common misinterpretations are flawed. We also show how RAP is profoundly sensitive to the specification of disease dependence on age. We conclude that whereas RAP remains a potentially useful measure, it requires multiple cautions beyond those needed for traditional measures of association, especially when computed for rates.