A measure of the frequency of occurrence of a phenomenon. In epidemiology, demography, and vital statistics, a rate is an expression of the frequency with which an event occurs in a defined population, usually in a specified period of time. The components of a rate are the numerator, the denominator, the specified time in which events occur, and usually a multiplier, a power of 10, that converts the rate from an awkward fraction or decimal to a whole number.

In vital statistics,

In epidemiology, the denominator is usually Person-time. Physical units other than time may be used for constructing rates; e.g., in accident epidemiology, deaths per passenger-mile is a more meaningful way of comparing modes of transportation.

Other uses of rate in epidemiology include: (1) As a wrong synonym for ratio, it refers to proportions as rates, as in the terms cumulative incidence rate or survival rate. Proportion and Ratio are not synonyms for rate. (2) In other situations, rate refers only to ratios representing relative changes (actual or potential) in two quantities. (3) Sometimes rate is further restricted to refer only to ratios representing changes over time. In this sense, the term prevalence rate is to be avoided, because Prevalence cannot (and does not need to) be expressed as a change in time; of course, different prevalence estimates may vary, change, and be compared. In contrast, the force of mortality and the force of morbidity (hazard rate) are proper rates, for they can be expressed as the number of cases developing per unit time divided by the total size of the population at risk.^{1, 2, 3, 5, 197} See also hazard rate.