What is the size of the population at time t + 1 ?
Populationt + 1 = Populationt + Naturalincreaset + Netmigrationt
Natural increase from time t to t + 1:
Natural Increaset = Birthst − Deathst
Net migration from time t to t + 1:
Netmigrationt = Immigrationt − Emigrationt
This basic equation can also be applied to subpopulations. For example, the population size of ethnic groups or nationalities within a given society or country is subject to the same sources of change. However, when dealing with ethnic groups, "net migration" might have to be subdivided into physical migration and ethnic reidentification (assimilation). Individuals who change their ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another.
More generally, while the basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made.