Specification and Implementation of Age, Period and Cohort Models

For the past 80 years or more, social scientists have attempted to analyze cross-time data, using as explanatory variables age and time (or phenomena that are time-specific). When such data are analyzed in aggregate forms, age and time are typically grouped and polytomized. More recently, some investigators have adopted an analytic focus in which cohort membership, as defined by the period and age at which an individual Observation can first enter an age-by-period data array, is held to be more important than age or period for substantive understanding. This focus has led to age-cohort and period-cohort models, as distinguished from age-period models.