The measures of compositions of populations are usually the most interesting. Demographers study age compositions, usually by age groups. They can also study the dependency ratio , which is the number of children and elderly compared to the number of working-aged adults. Another measure is the children to elderly ratio. By observing this ratio, demographers can tell if a population is young or old, and what implications it has on future social programs.
Demographers also study sex composition, like the sex ratio .
This ratio tells of the number of men to women, which is especially important in the age-groups when populations begin marriage. Lastly, there are measures of race composition. Study changes in race composition can sometimes show the levels of migration which is occurring within a population. Demographers study migration into a population and out of a population. Sometimes, demographers will also study the migration within a population, to study the mobility of the people.