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. There are many other measures which demographers study, which are more detailed and specific for reasons of particular interest or research. These measures all describe a population, and when studied over time, can define the trends within a population. This, to me, is what demography entails.