Parity progression ratios

A parity progression ratios (PPR) is a measure commonly used in demography to study fertility. The PPR is simply the proportion of women with a certain number of children who go onto have another child. Calculating the PPR, also known as a x , can be achieved by using the following formula:

a x = (women with at least  x + 1  children ever borne)  /  (women with at least  x  children ever born)

In more developed countries where two child families are seen as the norm a 2 (or the proportion of women with two children who go on to have a third) is of critical importance in determining overall fertility levels.

a 0 is simply the proportion of women who become mothers.