Parity Progression Ratios
The most widely used measures of fertility are period measures - age-specific fertility rates and the total fertility rate (TFR). These are very useful measures but they do suffer from one failing - they are not very good at detecting real changes in fertility in the short term. That is because they are affected by the timing of births (tempo effects).
In developed countries one cannot be completely sure, for several years, that an apparent decline in fertility is not at least partially due to delaying of births until older ages, to fit in with careers and a fashion for establishing households and financial security before childbearing. Of course there is often a concurrent real fertility decline as well, not least because delaying childbearing until later ages reduces the time in which to conceive and bear many children, and takes some women past periods of optimum fecundity. But even if women end up having the same number of children as women of an earlier cohort (i.e fertility remains constant), delaying having those births will produce an apparent drop in fertility as measured by period rates, at least for a while.
In developing countries too a phenomenon known as the Potter effect can suggest fertility declines that are in fact unreliable. In cultures where dates of birth are not considered important - and therefore children’s ages are not accurately known - women asked in surveys to report dates of birth of recent borne children frequently push the date of birth back in time; that is, they report children as older than they really are. Researchers calculating period fertility rates from these reports invariably see a declining trend in period fertility in the years leading up to the survey. This decline is of course false, only occurring because of misreporting of dates, and raises false hopes of a real decline in fertility.
Period fertility is therefore affected by two different components of fertility - the timing of births, which demographers call tempo effects, and the real level of (cohort) fertility, as measured by how many births a woman eventually produces, which demographers call the quantum of fertility.
It would therefore be useful to have measures of fertility that are largely insensitive to tempo effects. Parity progression ratios go a long way to meeting this need, and are also robust to misreported dates of birth. They can be very useful when trying to establish evidence for real fertility decline as distinct from apparent fertility decline produced by tempo effects on period measures.