Tempo distortions in the period TFR
The overall period TFR is susceptible to tempo changes in the timing of childbearing just as much as birth order-specific fertility rates. This is illustrated in greater detail in Table 2. The data in Table 2 are age-specific fertility rates for a hypothetical population with a TFR of about 3.0 children per woman. The period TFR is computed by summing the rates in the columns (example in red). The cohort TFR can be obtained by summing the ASFRs along the diagonals (example in blue).
There are a couple noteworthy issues in Table 2. First, one may note that the cohort TFR can only be computed for 5 cohorts of women (starting with the cohort of women that reach age 15 in t0 and ending with the cohort reaching age 15 in t20). Second, the cohort TFR stays constant at 3.0, indicating that the quantum of fertility does not change for these 5 cohorts of women. What changes, however, is the timing of their childbearing: starting with the cohort that reaches age 15 in t10 (blue diagonal), women postpone childbearing (ASFRs under age 30 are lower than in previous cohorts) and recuperate the foregone births later in life (ASFRs above age 30 are higher than before). The women who reach age 15 in t20 revert back to the old age schedule of fertility. Compared to the two previous cohorts, they thus advance their childbearing. These changes in tempo produce (artificial) fluctuations in the period TFR, which drops below 3.0 in the years that young women start to postpone childbearing and exceeds 3.0 in the periods that women advance their childbearing.
Note that these tempo changes are related to changes in the Mean Age at Childbearing (MAC) in the cohort: MAC in the cohort reaching age 15 in t5 is 28.17 years and increases to 30.92 years in the cohort starting childbearing in t10.
| Age | t0 | t5 | t10 | t15 | t20 | t25 | t30 | t35 | t40 | t45 | t50 |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 15-20 | 80 | 80 | 40 | 40 | 80 | 80 | 80 | 80 | 80 | 80 | 80 |
| 20-25 | 150 | 150 | 150 | 100 | 100 | 150 | 150 | 150 | 150 | 150 | 150 |
| 25-30 | 140 | 140 | 140 | 140 | 150 | 150 | 140 | 140 | 140 | 140 | 140 |
| 30-35 | 120 | 120 | 120 | 120 | 120 | 130 | 130 | 120 | 120 | 120 | 120 |
| 35-40 | 70 | 70 | 70 | 70 | 70 | 70 | 100 | 100 | 70 | 70 | 70 |
| 40-45 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | 60 | 60 | 30 | 30 |
| 45-50 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 20 | 20 | 10 |
| Cohort TFR | 3.00 | 3.00 | 3.00 | 3.00 | 3.00 | ||||||
| Period TFR | 3.00 | 3.00 | 2.80 | 2.55 | 2.80 | 3.10 | 3.20 | 3.30 | 3.20 | 3.05 | 3.00 |
Table 2: Period and Cohort TFR in a population with tempo changes in childbearing