I have kinda puzzled when I heard from the other about the calculation of incidence rate
From Kenneth Rothman's Modern Epidemiology, the incidence rate is calculated as number of cases in a fix period of time divided by person-time at risk at that period of time, which mean if the patient has the disease in the middle of the month, only the first half of patient days will be included in the denominator, but not for the second half. Am I correct?
Problem arises when I saw someone use the following formula for calculation:
number of cases in a fix period of time/total person-time at the period of time
If this is used, then the incidence rate will be underestimated, because the denominator now is bigger than it should be. But this formula seems to be used often, I wonder what this formula does. Thanks!
Best Answer
It is commonly admitted that the denominator for IRs is the "population at risk" (i.e., all individuals in which the studied event(s) may occur). Although your first formula is generally used, I found in The new public health, by Tulchinsky and Varavikova (Elsevier, 2009, 2nd. ed., p. 84) that a distinction is made between ordinary incidence rate, where the average size of the population in the fixed period of time is used in the denominator, and person-time incidence rate, with PT at risk in the denominator.
Obviously, when individuals not at risk of the disease are included in the denominator, the resultant measure of disease frequency will underestimate the true incidence of disease in the population under investigation, but see Numerators, denominators and populations at risk.