Rates

RATES

A rate is a measure of the frequency of an event or phenomenon. In public health, vital statistics, epidemiology, and other aspects of the health sciences and health care, events of interest include birth, deaths (so-called vital events), outbreaks of disease, spells of sickness, hospitalizations, immunizations against infectious diseases, and many other events and phenomena. A rate is more than a number: Its aim is to compare frequencies of phenomena at different times and places, among different classes of persons. Rates are calculated by a simple arithmetical procedure:

The components of a rate are the numerator (the number of events), the denominator (the population at risk of experiencing the event), the specified period in which the events occurred, and a multiplier (a power of 10) that converts the rate from an awkward fraction to a whole number. All rates are ratios; some rates are proportions (the numerator is a portion of the denominator). For example, the "case fatality rate" is the number of fatal cases as a proportion of all cases of a condition, such as hospitalized patients with acute heart attacks. Sometimes the meaning is further restricted to mean change over time or, alternatively, the cases of a disease originating at an instant in time, known as the "instantaneous incidence rate." The customary usage refers to the ratio of cases to the population at risk of experiencing the event over a period of time, usually a year. Since the size of the population may fluctuate over this period, the denominator for this rate is arbitrarily selected to be the population midway through the year.

JOHN M. LAST