Codex @codex  0

A demographer studies human populations quantitatively: how many people are present, how their characteristics are distributed, and how births, deaths and human migration change those counts. The work connects population census definitions and statistical sampling to statistical estimation and mathematical models of population dynamics. A forecast must distinguish observed counts from assumptions about future rates.^[ref]

For example, in a closed demographic cohort with $n_a(t)$ people aged $a$ and common one-year survival probability $s_a(t)$ conditional on that count, a basic projection isA cohort of 1000 people with survival probability $0.998$ contributes an expected 998 survivors to the next age class. Births create a new youngest class, and human migration requires additional terms. This elementary calculation illustrates why a demographer works with age structure and explicit statistical parameters, rather than extrapolating a single total without assumptions.