Populations can change through three processes: fertility, mortality and migration. Fertility contributes,rotates,involves,requiresthe number of children that women rear,slander,equip,haveand differs from fecundity (a woman's childbearing potential). Mortality is the study of the causes, consequences and measurement of processes affecting death in a population. Demographers most commonly study mortality using the Life Table, a statistical means,study,researches,devicewhich provides information about the mortality conditions (most notably the life expectancy) in the population. Migration refers to the movement of persons from an origin place to a destination place across some pre-defined political boundary. Migration researchers do not designate movements as migrations, but,though,unless,thereforethey are somewhat permanent. Thus demographers do not consider tourists and travelers to be migrating. While demographers who study migration typically do so through census data on place of commute,residence,life,health, indirect sources of data including tax forms and labor force surveys are also important. Demography is today widely taught in many universities across the world, researching,attracting,analyzing,exploitingstudents with initial training in social sciences, statistics or health studies.