Review of cohort PPRs
Parity progression ratios (PPRs) measure the probability that a woman will have a further birth in relation to, not her age, but how many children she has given birth to already. PPRs are usually denoted an, where the subscripted index refers to the number of children that the women have borne already.
- a0 = proportion of childless women who have 1+ children (i.e. become mothers)
- a1 = proportion of women with 1 child who go on to have 2+ children
- a2 = proportion of women with 2 children who go on to have 3+ children, etc.
A PPR, an, of 0 would mean that no women with n children had progressed to have an n+1th birth while, at the opposite extreme, a parity progression ratio of 1 would imply that all women with n children progressed to give birth to another child.