Incidence density is calculated by dividing the number of new cases of a disease by the total person-time at risk. Person-time combines the number of people at risk with the amount of time they were at risk. The formula is: Incidence Density = (Number of new cases) / (Total person-time at risk) This method accounts for individuals entering and leaving the study at different times, thus providing a more accurate measure of disease frequency in dynamic populations.
