Age distribution and the stable equivalent

Some populations, like that of the United States in the 1950's, have a smaller proportion of women of reproductive age than they would ultimately attain with continuance of their age-specific birth and deaths rates, a continuance which produces the condition known in demography as stability. Others, like that of the United States in the 1930's, have relatively more women of reproductive age than they would ultimately attain with stability. A way of studying ages is to calculate how many women of stable age distribution would be equivalent from the viewpoint of reproduction to the women observed. This stable equivalent was 69,535,000 or 16 percent below the observed United States female population in 1955, and 12 percent above the observed in 1935. The stable equivalent is a measure of fertility potential, closely related to R. A. Fisher's reproductive value. Calculations for four countries illustrate how a fall of the birth rate, for example in demographic transition, occasions an age distribution in which the stable equivalent is greater than the observed number of women. The notion of stable equivalent is useful for comparison because changes in it are nearly invariant with respect to the age-pattern of fertility used. The statement that the United States stable equivalent increased by 11 percent between 1960 and 1965 holds irrespective of whether the 1960 or the 1965 age-specific fertility and mortality rates are used as standard.