  Social scientists have frequently sought to understand the distinct effects
of age, period, and cohort, but disaggregation of the three dimensions is
difficult because cohort = period - age. We argue that this technical
difficulty reflects a disconnection between how cohort effect is conceptualized
and how it is modeled in the traditional age-period-cohort framework. We
propose a new method, called the age-period-cohort-interaction (APC-I) model,
that is qualitatively different from previous methods in that it represents
Ryder's (1965) theoretical account about the conditions under which cohort
differentiation may arise. This APC-I model does not require problematic
statistical assumptions and the interpretation is straightforward. It
quantifies inter-cohort deviations from the age and period main effects and
also permits hypothesis testing about intra-cohort life-course dynamics. We
demonstrate how this new model can be used to examine age, period, and cohort
patterns in women's labor force participation.
