This article looks at a problem that has been studied over decades: the unidentifiability issue in the age-period-cohort (APC) model. The impossibility to identify the full set of age, period, and cohort effects in the analysis of mortality or other vital rates has created much confusion in the epidemiological and sociological literature. The misconception that the problem is a shortcoming of current statistical methods still seems to be prevalent, and apparently a new “solution” has been proposed recently: the intrinsic estimator (IE).
However, the difficulty is inescapable and arises from the fact that subjects cannot move in one time scale without an identical move in others (Clayton and Hills 1993:315). In order to develop some intuition for this phenomenon, it helps to visualize the simultaneous development of age, period, and cohort in a Lexis diagram. Similar graphical devices date back to sociological literature from the 1870s published in German;...
