Among the kinds of data that national leaders need are the demographic statistics of their population. Records of births, deaths, marriages, immigration and emigration and a regular census of population provide information that is key to making sound decisions about national policy.
A useful summary of such data is the population pyramid. It provides data about the sex and age distribution of the population in an accessible graphical format.
Another summary is called the life table. For a cohort of persons born in the same year, it traces and projects their life experiences from birth to death. For a given cohort, the proportion expected to survive each year (or decade in an abridged life table) is presented in tabular or graphical form.
The ratio of males to females by age indicates the consequences of differing mortality rates on the sexes. Thus, while values above one are common for newborns, the ratio dwindles until it is well below one for the older population.