Ryder’s translation

Norman Ryder also recognized that the period TFR is susceptible to tempo changes in childbearing, and developed an approach for recuperating the period TFR from the cohort TFR and the Mean Age at Childbearing (MAC) in a cohort:

TFR=CFS(1−c)

Where c is that annualized rate of change (i.e., the time derivative) in the cohort MAC, and can be estimated as:

C= MA C t+1 −MA C t−1 2

This equation demonstrates that a cohort’s postponement of childbearing (c > 0) leads to a decline in the period TFR by an amount that depends on the rate of change in the cohort MAC. The period TFR, in this equation pertains to period or year in which the cohort reaches its MAC.

Using this approach, Ryder argued that a substantial proportion of the post WWII baby boom could be attributed to declines in the MAC (c < 0), or in other words, to tempo changes in fertility. Ryder’s approach has proven useful to retroactively diagnose tempo distortions in the period TFR, but it is much less practical to either assess or correct tempo effects in contemporaneous period fertility estimates. One measure that attempts to do precisely that is the tempo-adjusted total fertility rate, TFR*, introduced in 1998 by John Bongaarts and Griffith Feeney.