Inverse Projection

Inverse projection, and its more flexible successor generalized inverse projection, reconstructs the demographic history of a population from the outside in. Where conventional cohort-component projection runs a known population forward using assumed rates, inverse projection runs the logic backward: starting from a population of known size and age structure at one date, and given annual totals of births and deaths, it infers the population sizes, age distributions, mortality levels, life expectancies and fertility rates that must have prevailed in earlier years. The technique was devised by Ronald Lee and applied by Wrigley and Schofield to their English aggregative series, allowing three centuries of population history to be recovered without any direct census before 1801. Jim Oeppen's generalization relaxed restrictive assumptions about migration and closed populations. The method is the bridge that turns raw counts of vital events into a fully articulated demographic regime.