The period foregrounds cohort-based measurement as a unifying framework for translating population processes into observable statistics. Methodological advancement centers on cohort analysis, capture-recapture adaptations, and stratified modeling that jointly address fertility, mortality, migration, and survival across heterogeneous populations. Emphasis is placed on robust, nonparametric and model-based estimators that contend with heterogeneity in capture probabilities and structured population dynamics, guiding both theory and applied measurement in demography and related fields.