Timing Effects and the Interpretation of Period Fertility

Robert Schoen, Pennsylvania State University

Low fertility levels and later childbearing in many developed countries have reinvigorated the period/cohort debate on the meaningfulness of the period TFR. Here, timing effects are defined as level changes in period fertility that do not reflect changes in completed cohort fertility, and the Average Cohort Fertility (ACF) is used as a measure of period fertility adjusted for timing effects. Bongaarts and Feeney (1998) presented an alternative approach and a different measure, TFR*, to adjust for timing effects. Here the two measures are compared. Conceptually, the TFR* is based on questionable assumptions. In model populations, the ACF performs as expected, while the Bongaarts-Feeney TFR* is unreliable and often erratic. When applied to 20th century U.S. experience, the ACF moves gradually, quantifying the substantial timing effects present during the 1970s "Birth Dearth." In contrast, the TFR* behaves like a period measure, and its adjustments are often wide of the mark.