Demography is a subspecialty within sociology that focuses on the study of human populations, particularly on their size and rate of growth. Population size is determined by births, deaths, age-sex composition, and migration. Most organizations—private, public, and governmental—have an interest in knowing population characteristics, if only for planning purposes. For example, school officials need to know the size of the school-age population and whether it is projected to decline or increase, as this will affect decisions to expand or consolidate the number of schools.
Health care planners need to know the size of the population age 65 and older and whether it is projected to decline or increase, as this age group has some of the greatest health care needs. As a final example, city planners need to know the size of the population and whether that population is expected to decline or increase as the result of in- or out-migration.
The crude birth rate is the annual number of births per 1,000 people in a designated area. Births add new people to a population. Each year, India adds through births approximately 27 million people and the United States adds about 4.2 million. In 2007 India's crude birth rate was 24 per 1,000 population the United States' rate was 14 per 1,000. Sometimes demographers want to know the birth rate for a specific age group within the population. The birth rate among women of childbearing age (15–54 years old) is called the age-specific birth rate. In 2007, the 300.5 million women of child- bearing age in India gave birth to 27,116,788 babies. The age-specific birth rate for this group was 90.2 per 1,000 women. In the United States, the age-specific birth rate was 43.5 per 1,000.
The total fertility rate states the average number of children that women in a specific population bear over their lifetime. The average woman in India bears 2.6 children over her lifetime, whereas the average woman in the United States bears 2.1 children.
The crude death rate is the annual number of deaths per 1,000 people in a designated area. The death rate among children one year old or younger is called the infant mortality rate. In the United States, the infant mortality rate is approximately 6 per 1,000; in India the rate is 40 per 1,000.
Migration is the movement of people from one residence to another. That movement increases a population if the people are moving in, reduces the population if they are moving out, or makes no difference if they are simply moving within the geographic area of interest. Each year about 56,500 more people leave India than move into the country. Migration results from two factors. Push factors are the conditions that encourage people to move out of an area. Common push factors include religious or political persecution, discrimination, depletion of natural resources, lack of employment opportunities, and natural disasters. Pull factors are the conditions that encourage people to move into an area. Common pull factors include employment opportunities, favorable climate, and tolerance.