The Total Fertility Rate (cont.)
Period fertility vs. cohort fertility
The following table lists the main contrasts between the period and cohort approach to measuring fertility. Both have their own uses and applications and give a different view of the quantum of fertility. Together they allow a more rounded view of what is happening to fertility. Period fertility is by far the more commonly reported because it is “instant” and better understood by those without demographic training – so it becomes the default. However cohort fertility analysis frequently allows a more comprehensive picture to emerge.
| Period fertility | Cohort fertility |
|---|---|
| Relates to short intervals of time | Relates to lifetime experience |
| Looks at births to women in all age groups | Follows real women over their reproductive life-times |
| Can produce very current estimates | Estimates typically relate to an earlier period |
| Data can be collected in a single time period or in a cross-sectional survey. For recent childbearing (last 10 years) recall of dates of birth is usually quite good | Requires either longitudinal data or retrospective questioning by means of a birth history from women aged 50+. In populations not conversant with recording of dates the latter may suffer from inaccuracies of age/date recall for distant births |
| Summary measure is Total Fertility Rate (TFR) | Summary measure is Completed Family Size (CFS) |
| Refers to an artificial construct called a synthetic cohort | Reflects the lifetime behaviour and intentions of cohorts of real women |
| Hhighlights the yearly variations in fertility due to transient influences | Smooths out temporal variations in fertility, since real women may live through periods of high and low fertility |
| Parity progression analysis complicated | Easy to describe family formation in terms of parity progression* |
| Best way to study impact of crises and short term interventions | Best way to study childhood influences on childbearing outcomes |
*See next section