Incidence is typically expressed as an incidence rate or incidence proportion. The incidence rate is calculated by dividing the number of new cases by the person-time at risk (often presented per 1,000 or 100,000 person-years). The formula is:
Incidence Rate = (Number of New Cases) / (Total Person-Time at Risk)
The incidence proportion, also known as cumulative incidence, is the number of new cases divided by the total population at risk during a specified period. The formula is:
Incidence Proportion = (Number of New Cases) / (Population at Risk)
