 Human population

A human population for quantitative demography is the collection of people satisfying a stated territorial, residence and time definition. Its size $N(t)$ is a count; its age or other composition describes how that count is distributed between subgroups. A resident population and the people physically present on a particular night can have different sizes.

For example, a town with 10000 residents and 200 births during one year has an approximate crude birth rate of 20 births per 1000 residents per year if its midyear population is 10000. A town with 20000 residents and the same birth count has approximately half that rate. Raw event counts therefore cannot replace denominators. Population censuses and the demographic balancing equation connect successive measurements, while demographic cohorts allow changes in composition to be tracked.