Editing Demography (section)

==Basic equations for regional populations==
Suppose that a country (or other entity) contains ''Population<sub>t</sub>'' persons at time ''t''.
What is the size of the population at time ''t'' + 1 ?

:<math>\text{Population}_{t+1} = \text{Population}_t + \text{Natural Increase}_t + \text{Net Migration}_t</math>

Natural increase from time ''t'' to ''t'' + 1:

:<math>\text{Natural Increase}_t = \text{Births}_t - \text{Deaths}_t</math>

[[Net migration rate|Net migration]] from time ''t'' to ''t'' + 1:

:<math>\text{Net Migration}_t = \text{Immigration}_t - \text{Emigration}_t</math>

These basic equations can also be applied to subpopulations.  For example, the population size of ethnic groups or nationalities within a given society or country is subject to the same sources of change.  When dealing with ethnic groups, however, "net migration" might have to be subdivided into physical migration and ethnic reidentification ([[Cultural assimilation|assimilation]]).  Individuals who change their ethnic self-labels or whose ethnic classification in government statistics changes over time may be thought of as migrating or moving from one population subcategory to another.<ref>See, for example, Barbara A. Anderson and Brian D. Silver, "Estimating Russification of Ethnic Identity Among Non-Russians in the USSR," ''Demography'', Vol. 20, No. 4 (Nov., 1983): 461-489.</ref>

More generally, while the basic demographic equation holds true by definition, in practice the recording and counting of events (births, deaths, immigration, emigration) and the enumeration of the total population size are subject to error. So allowance needs to be made for error in the underlying statistics when any accounting of population size or change is made.

The figure in this section shows the latest (2004) UN ([[United Nations]]) [[World Health Organization|WHO]] projections of [[world population]] out to the year 2150 (red = high, orange = medium, green = low). The UN "medium" projection shows world population reaching an approximate equilibrium at 9&nbsp;billion by 2075. Working independently, demographers at the [[International Institute for Applied Systems Analysis]] in [[Austria]] expect world population to peak at 9&nbsp;billion by 2070.<ref>{{cite journal|url=http://ace1.ma.utexas.edu/users/davis/375/reading/worldbirthrate.pdf |title=Doubling of world population unlikely |first=Wolfgang |last=Lutz |author2=Sanderson, Warren |author3=Scherbov, Sergei |journal=Nature |volume=387 |pages=803–805 |date=1997-06-19 |access-date=2008-11-13 |doi=10.1038/42935 |pmid=9194559 |issue=6635 |bibcode=1997Natur.387..803L |s2cid=4306159 |url-status=dead |archive-url=https://web.archive.org/web/20081216230409/http://ace1.ma.utexas.edu/users/davis/375/reading/worldbirthrate.pdf |archive-date=16 December 2008 |df=dmy }}</ref> Throughout the 21st century, the average age of the population is likely to continue to rise.