Measure of the frequency with which an event occurs in a defined population in a specified period of time.

Note 1: Most such rates are ratios, calculated by dividing a numerator, e.g. the number of deaths, or newly occurring cases of a disease in a given period, by a denominator, e.g. the average population during that period.

Note 2: Some rates are proportions, i.e. the numerator is contained within the denominator (as when a number of patients with a given disease is divided by the total population from which they come).