Demography is the statistical and mathematical study of the size, composition, and spatial distribution of human populations and how these features change over time. Data is obtained from a census of the population and from registriesrecords of events like birth, deaths, migrations, marriages, divorces, diseases, and employment. To do this, there needs to be an understanding of how they are calculated and the questions they answers which is included in these four concepts: population change, standardization of population numbers, the demographic bookkeeping equation, and population composition. Population change is analyzed by measuring the change between one population size to another. Global population continues to rise, which makes population change an essential component to demographics. This is calculated by taking one population size minus the population size in an earlier census. The best way of measuring population change is using the intercensal percentage change. The intercensal percentage change is the absolute change in population between the censuses divided by the population size in the earlier census. Next, multiply this by 100 to receive a percentage. When this statistic is achieved, the population growth between two or more nations that differ in size, can be accurately measured and examined. For there to be a significant comparison, numbers must be altered for the size of the population that is under study. For example, the fertility rate is 10 calculated as the ratio of the number of births to women of childbearing are to the total number of women in this age range (multiplied by 1000). If these adjustments were not made, we would not know if a nation with a higher rate of births or deaths has a population with more women of childbearing age or more births per eligible woman. Within the category of standardization, there are two major approaches: direct standardization and indirect standardization. Direct standardization is able to be used when the population being studied is large enough for age-specific rate are stable. Indirect standardization is used when a population is small enough that the number of events (births, deaths, etc.) are also small. In this case, methods must be used to produce a standardized mortality rate (SMR) or standardized incidence rate (SIR) Population composition is the description of population defined by characteristics such as age, race, sex or marital status. These descriptions can be necessary for understanding the social dynamics from historical and comparative research. This data is often compared using a population pyramid. Population composition is also a very important part of historical research. Information ranging back hundreds of years is not always worthwhile, because the numbers of people for which data are available may not provide the information that is important (such as population size). Lack of information on the original data-collection procedures may prevent accurate evaluation of data quality.