Population change is simply the change in the number of people in a specified area during a specific time period. Demographics (or demography) is the study of population statistics, their variation and its causes. These statistics include birth rates, death rates (and hence life expectancy), migration rates and sex ratios. All of these statistics are investigated by censuses and surveys conducted over a period of time. Some demographic information can also be obtained from historical maps, and aerial photographs.
A major purpose of demography is to inform government and business planning of the resources that will be required as a result of population changes.