"Demography" is the name for the social science which concerns itself with studying populations. Specifically, it is the science of the social statistics of populations, such as births, deaths, diseases and marriages.
The word comes from "demos," an Ancient Greek word meaning "the people," and "graphy," which means "measurement." Demography involves studying the size, structure and distribution of people and how things, such as birth, migration, aging and death, change population over time. There are two ways of gathering demographic data. Direct data is the first method; it involves statistics which are directly collected, such as registries and surveys. When this is not available, indirect methods are used.