Latest revision as of 05:54, 5 February 2010

Age-specific rates ¹ are computed for single years of age or for age groups (age-group specific rate ^2★ or age group-specific rate ^2★). Duration-specific rates ³ take into account the time elapsed since a baseline event ⁴ or event-origin ⁴ such as marriage or a previous birth. Central rates ¹⁰ are obtained by dividing the number of events during a year, or some other period (often five years) either by the average population ⁶ or mid-year population ⁶ or by the number of person-years ⁷ of exposure to the event in question during that year or period; the number of person-years is the sum, expressed in years, of the exposure time for all individuals in the observed group, over the year or period. The term rate is sometimes used also for another type of measure, obtained by dividing the number of non-renewable events in a year or a period of years by the size of the cohort considered at the beginning of the year or period; this measure is sometimes called an attrition probability ⁵ or more simply a probability ⁵, and contrasted with the central rate, defined earlier. In this paragraph, the word "period" has referred to a length of time. In the expression period rates ⁸, however, the word is used in its chronological meaning and refers to a specific calendar year or group of years; it is opposed to cohort rate ⁹ or generation rate ⁹.

• 5. The word quotient, used in French for this type of rate, has sometimes been used in English.

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