= 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.
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