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Demographic variables are those variables that directly determine changes or directly drive growth in population size and structure. The variables include age and sex distribution, fertility (and its direct determinants such as marriage, contraceptive practice), mortality (and its direct determinants such as nutrition, environmental contamination, illness control) and migration (and its determinants such as economic and political environment factors). Income and education are not demographic variables but socio-economic variables.

Population variables are a wide range of socio-economic and environmental variables. There may be a two-way relationship between demographic and population variables. Population variables operate indirectly to influence demographic variables. Education (a social variable) for example, does not directly influence fertility but may operate through contraceptive practice to influence fertility.

A population projection is an estimate about the future size and structure of a population based on certain assumptions about fertility, mortality and net migration trends. A population forecast is an accurate prediction about the future size and structure of a population. To say that one is making a
population forecast or prediction population (as some researchers claim) is misleading the public because no one can do that since no one possesses a crystal ball regarding the future course of fertility, mortality and net migration. Although population scientists attempt to make realistic assumptions when undertaking population projections, the outcomes of such estimates will always have some errors. The quality of the underlying data would be one source of such errors. All sources of demographic and population data (census, surveys and vital registration) in any country will always have some errors.

Fertility is the actual performance of reproduction, thus refers to the number of live births of women. This is different from fecundity – the physiological ability to reproduce. Still births are not included as part of fertility.

Mortality refers to death.

Migration refers to movement of people across administrative boundaries with the intention of living in the place moved to. It is different from tourism or commuting from one city/town to the other for the purpose of work. Migration has two components – internal (migration across administrative boundaries within a country) and international (migration across international boundaries involving two different countries). Furthermore, both internal and international migration have two components: in- and out-migration in the case of internal migration; immigration and emigration in the case of international migration. The balance between these opposite movements (net-migration) along with fertility and mortality have a direct influence on population growth.

Every form of sectoral planning in any population is related to some age group. In the education sector for example, it is important to know the size of the school age children in order to plan effectively for the demands of this segment of the population about education. In the health sector, it is important to know the size of the number of children under the age of five years to enable efficient planning for vaccine programme and implementation for this segment of the population. Planning in the economic sector for example, job creation among others, requires accurate information about the size of the population aged 15-64, the labour market.