On the Pattern of Cohort Fertility.

Demographers have attempted to graduate schedules of age-specific fertility rates since the 1930's and there are now several functions which could be fitted to these rates. But little work has been done to date to construct model schedules of fertility. An exploratory study to examine the possibility of fitting certain mathematical formulae to cumulative fertility rates of marriage cohorts in England and Wales revealed the existence of a common pattern showing how mean family sizes of the different age-at-marriage groups of a given marriage cohort, are built up over the childbearing span. To test the adequacy of the generated system for describing observed fertility distributions, the researchers have estimated the distribution of mean family sizes by age at and duration of marriage, for a selection of marriage cohorts of completed fertility in England and Wales and Sweden using various methods. The adequacy of the generated system to describe widely different childbearing patterns suggests that the researchers have a useful tool for fertility projections because, if they are prepared to accept the idea that "a marriage cohort carries its own fertility pattern with it", it follows that changes in fertility patterns consist mainly of a shift of the location of the cumulative fertility curve on the standardized vertical and horizontal scales.