Population size is determined by a balance between birth rates, death rates, immigration, and emigration. Population change is measured as the difference in population size between the beginning and end of a time period, usually one year. The crude birth rate and crude death rate measure live births and deaths per 1,000 people respectively, and natural population change is calculated as births minus deaths. Actual population change also takes into account net migration. Factors like fertility rates, mortality rates, and life expectancy provide more detail on changes in birth and death rates over time.