Fertility change: Quantum and tempo
The ‘quantum’ and ‘tempo’ dimensions of fertility usually refer, respectively, to the average number of births per woman during the reproductive years and the average age at childbearing. These measures can be estimated for cohorts of women who have completed their childbearing years, as well as for specific time periods. Unfortunately, quantum measures of period fertility (e.g., the total fertility rate) can be distorted, because its quantum and tempo dimensions interact. Assessment of period fertility levels and trends therefore should include adjustments to remove tempo distortions caused by changes in the timing of childbearing.
Bongaarts, John. 2015. "Fertility change: Quantum and tempo," in James D. Wright (ed.), International Encyclopedia of the Social & Behavioral Sciences (Second Edition). Oxford: Elsevier.