The Population Pyramid shows the balance between generations. Population aging has become a problem in much of the world. Changes in birth or death rates affect a population on the tim e scale of a generation. Populations respond to the pressures caused by natural increase in a variety of ways. Joel Cohen reviews a surprisingly wide range of answers.
Censuses provide a snapshot of a population at a point in time.
Censuses require a national administrative capability that can be mobilized over a short period of time. Vital registration requires an effective bureaucracy and a cooperative population, and these data are difficult to use when registration incomplete. Sample surveys can be used to measure certain kinds of demographic rates in the absence of vital registration. These techniques provide reliable estimates of demographic rates in populations that have neither censuses nor vital registration.