A method of estimating or projecting the population by updating the size of each age-sex group in the base population for deaths and migration within each age-sex group during the period between the base date and a given date. New birth cohorts result from births between the base date and the given date.
In deriving population projections, the base population is projected forward by calculating the effect of deaths and migration within each age-sex group according to specified mortality and migration assumptions. New birth cohorts are generated by applying specified fertility assumptions to the female population of childbearing age.
In deriving population estimates, actual events on births, deaths and migration are generally used instead of assumptions.en-NZ