= Demography
{wiki}
<Demography> measures the size, composition and change of <human populations>. It combines <population censuses>, registrations of births and deaths, and <statistical sampling> with <mathematical models> of <population dynamics>. A <population census> refers its counts to a specified time and territory; collecting questionnaires on different days should not silently change the reference date.<Principles and Recommendations for Population and Housing Censuses Revision 3 by the United Nations Statistics Division>[Paragraphs 1.4–1.12, census definition, universality and simultaneity]{ref}
For a fixed territory, a demographic accounting identity is
$$
N(t_1)=N(t_0)+B-D+I-E,
$$
where $B,D,I,E$ count births, deaths, immigration and emigration during the intervening period. For example, $N(t_0)=10000$, $B=140$, $D=90$, $I=80$ and $E=110$ give $N(t_1)=10020$. This <demographic balancing equation> distinguishes natural increase from net <human migration>; it does not assume an <exponential growth> law.
Counts also need denominators. A <birth rate> compares births with the relevant population exposure, rather than comparing raw birth totals between differently sized <human populations>. Changes in age composition can alter an aggregate rate even when the underlying age-specific rates remain fixed, motivating analysis by <demographic cohorts>.
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