It is easier to interpret risks than rates. Therefore, it might sometimes be convenient to convert an incidence rate into a risk using the following formula :
Risk = Rate x Time
Suppose that we have a population of 1000 persons in which the incidence rate ofcancer X is 6 cases per 1000 person-years (6 / 1000 yr-1). If we follow this population for 30 years the risk of cancer X in the population over that 30 years is: 6 / 1000 yr-1 x 30 years = 0.18 or 18%. Among the 1000 persons present at the start of the follow-up, 180 cases of cancer X will occur. If the follow up was 15 years the related risk would be 9 %. The above formula does not take into account the decrease of the population at risk over time and cannot be used when risk is large. It also assumes that rate remains constant over time.
1. Rothman KJ; Epidemiology: an introduction. Oxford University Press 2002, p.33-38.