Principles of Public Health Exam 1

Incidence rate is defined as the number of new cases that develop during a specific time period divided by the population at risk during that period. The key idea is that only people who could actually develop the disease contribute to the denominator. Those who already have the disease, are immune, or are otherwise not at risk are excluded because they cannot contribute to new cases. This is why the denominator should be the population at risk during the period.

The other options don’t fit: the number of deaths relates to mortality, not new cases; the total population includes people not at risk and would distort the rate; and the number of cases is the numerator, not the denominator. For example, if 12 new cases occur in 1,000 people who are at risk during a year, the incidence rate is 12/1,000 (per year), not 12 divided by the total population or by the number of existing cases.