Demographic measurement: general issues and measures of fertility

The article starts by outlining the types of demographic events and states that are routinely measured in demography. Demographic measures are designed to quantify the level, timing, and distribution of demographic phenomena. Their purpose is to allow comparison across time and space, and between subpopulations. They are required particularly because of the substantial variation in the frequency of demographic events and states by age, and because populations vary in size and composition with respect to age and other factors influencing the frequency of events and states. Measures may be crude or specific by one or more factors. To allow for differences in structure between populations, sets of specific rates may be generated. A single-figure index may be used to summarize such arrays of specific rates by a number of forms of standardization. Standardization may also be carried out in the absence of specific rates for one or more of the populations to be compared. The entry goes on to describe the principal measures of fertility currently in use. Like other demographic rates, fertility measures are of two types: those in which the denominator is confined to those at risk of the event in question (type 1), and those in which the denominator includes all persons, irrespective of whether they are at risk of the event (type 2). Synthetic or hypothetical cohort indicators of fertility are designed both to summarize the rates of a period and to estimate the consequences for a hypothetical cohort of women subject throughout their lifetime to the rates of a particular period. There are several ways in which such indicators can be generated. The advantages and disadvantages of synthetic cohort indicators are discussed. Measures of both level and timing of fertility are presented. The definitions of the principal fertility measures currently in use are set out in tabular for