The population changes from one year to the next. Natural change is the difference between births and deaths, and can be determined precisely from vital records. Net migration is the difference between the number of immigrants and the number of emigrants. It is estimated on the basis of available statistics.

Adjustments are sometimes made to establish overall consistency between census population figures and inter-census estimates of population change.

To estimate its relative scale, population change is often expressed in relation to the mean population of a given year. Rates of birth, death, natural increase and total variation are obtained in this way.