Demography is the scientific study of changing population size and structures. It analyses the structures of human populations by age, sex, place of residence, level of educational attainment, labour force participation and other essential demographic characteristics of individuals. Formal demography analyses these changes with the help of mathematical models, which also allow projections over several decades under certain assumptions about future fertility, mortality and migration. Demography also addresses comprehensively the determinants and consequences of these population changes including the impacts on society, the economy and the environment in all parts of the world.