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Stable age distribution

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Stable age distribution  (STABLE age distribution)

It can be shown that when a closed population (701-4) is subjected to constant age-specific fertility and mortality rates (631-8; 412-1) for a sufficiently long period of time, its annual rate of increase will tend to become constant. This constant rate of increase is called the intrinsic rate of natural increase1 or true rate of natural increase1, and a population .which has reached this stage is called a stable population2. The proportion of persons in different age groups in such a population will be constant, i.e., the population will have a stable age distribution3 which is independent of the initial age distribution4 and depends only on the fertility and mortality rates that obtained. Human populations never reach stability in practice, as fertility and mortality rates constantly change, but the computation of a stable population as a model may serve as an index of the growth potential5 of a set of age-specific fertility and mortality rates. A stable population in which the rate of natural increase is zero is called a stationary population6, in such a population the numbers in a given age group are equal to the integral of the survivorship function (431-3) of the life table taken between the upper and lower age limits of the group. A logistic population7 growing in accordance with the logistic law of growth is one in -which the rate of growth decreases as a linear function of the population already alive and which will tend asymptotically to an upper limit.

  • 2. The crude birth and death rates of a stable population, are called the Stable birth rate and the stable death rate respectively.