Demography is an area of study that is concerned with statistics and changes in human populations. Emphasis is placed on the size and distribution of the population and how these are influenced by factors such as birth rates, aging/death rates, and migration. Essentially the study of populations, demographic studies look at how different factors contribute to changing populations. An example of demography is the US census which takes place every ten years. Residents of the United States fill out a form that allows the number of residents to be counted. Other information is gathered such as age, income, and number of children among other general identifying characteristics.