Introduction
The analysis of fertility by means of parity progression ratios (PPRs) was introduced in session PAPP101_S04 
. PPRs measure the proportion of women with n children who go on to bear an n+1th child.
PPRs examine fertility as an incremental family building process. Changes in the PPRs for different parities provide important insights into patterns of fertility change. For example, in low-fertility populations, the trend in the proportion of women progressing to a third birth is a crucial determinant of total fertility and the long-term growth prospects of the population. Moreover, the initial onset of parity-specific family size limitation in a high fertility population tends to be concentrated at the middle-order parities. By examining these PPRs, the onset of fertility decline can often be detected some years before its impact on total fertility become evident.
The earlier session covered the calculation of cohort parity progression ratios for women in age cohorts that had completed childbearing. This session covers three related topics.
The first topic is the calculation of PPRs for parity cohorts as opposed to age cohorts. A parity cohort is a group of women who all have a birth of the same parity in the same period of time. Therefore they all become at risk of progressing to their next birth at the same time.
The second topic is the calculation of period PPRs. Whereas cohort PPRs are calculated based on the births occurring over several decades to a single cohort of women, period PPRs are based on the births occurring in a single period of time (often either one year or five years) to women from all the different cohorts that were capable of bearing children at that time.
Third, the session discusses measures of the length of birth intervals. PPRs measure the proportion of mothers that close birth intervals of each order by having another birth. To fully understand the dynamics of the family building, however, it is also important to measure how long it takes women to progress to their next birth according to their parity. Thus, an important aim of birth interval analysis is the calculation of summary measures such as the mean or median length of birth intervals.