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1 In vital statistics, demography, and epidemiology, a rate is a measure of the frequency with which specified events occur in a defined population in a designated period of time. In general terms, a rate is a measure of the frequency of occurrence of any phenomenon. Rates, rather than raw numbers, must be used in order to ensure that comparisons are meaningful. It is good practice to indicate the numerical values of the numerators and denominators on which rates are based. Rates have three components: a numerator, the number of events; a denominator, the numbers in the population at risk of experiencing the event; and a time dimension, the period over which the events occur.

Rate =

number of events in a specified period ¥ 10nnumber in population at risk of the event

The denominator can be expressed as person-time. Statisticians prefer to calculate an instantaneous incidence rate, the rate at which an event occurs in an instant of time. This is a theoretical concept and is rarely used in vital statistics. The word “rate” is loosely used in terms such as prevalence rate and survival rate. Point prevalence, the prevalence of a condition at a point in time, has no time dimension. The term “annual prevalence” can be confusing because conditions such as acute respiratory infection often occur more than once in a year. Survival rates are usually proportions, although they are often expressed with a time dimension, e.g., 5-year survival rates for breast cancer. Case fatality rate, an important indicator of the severity of a condition, is sometimes stated without a clear definition of the time dimension, merely with the numerator (number of deaths) and denominator (the population that has been exposed to the risk of dying).

2 The charge for a service is also called a rate, such as municipal rates, the charge for services such as street sweeping or collection of domestic waste.

Subjects: Public Health and Epidemiology.

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