One important area of study covers the components of population change or the factors responsible for change in the size of population. It must be understood that the population of any place at a specific time is a function of three types of events: births, deaths and migration.
There are four ways in which the number of people in any area can undergo change: (1) children may be born in that area; (2) the inhabitants of that area may die; (3) people from other areas may move into that area; and (4) inhabitants of that area may move out.
These components of population change, namely, births, deaths and migration are identified as fertility, mortality and migration respectively, and are known as demographic or population variables because the size, growth, structure and distribution of any population are determined by them. A study of any population is made through a study of these demographic variables.