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).
Subjects: Public Health and Epidemiology.