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Reverted edits by 112.198.230.103 (talk) (HG) (3.4.3)

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In [[statistics]], a '''population''' is a [[Set (mathematics)|set]] of similar items or events which is of interest for some question or [[experiment]].<ref>{{Cite web|title=Glossary of statistical terms: Population|website=[[Statistics.com]]|url=http://www.statistics.com/glossary&term_id=812|accessdate=22 February 2016}}</ref> A statistical population can be a group of existing objects (e.g. the set of all [[star]]s within the [[Milky Way]] [[galaxy]]) or a hypothetical and potentially [[Infinite set|infinite]] group of objects conceived as a generalization from experience (e.g. the set of all possible hands in a game of [[poker]]).<ref>{{MathWorld|Population}}</ref> A common aim of statistical analysis is to produce information about some chosen population.<ref>{{cite book | last = Yates | first = Daniel S. | last2 = Moore | first2 = David S | last3 = Starnes | first3 = Daren S. | year = 2003 | title = The Practice of Statistics | edition = 2nd | publisher = [[W. H. Freeman and Company|Freeman]] | location = New York | url = http://bcs.whfreeman.com/yates2e/ | isbn = 978-0-7167-4773-4 | deadurl = yes | archiveurl = https://web.archive.org/web/20050209001108/http://bcs.whfreeman.com/yates2e/ | archivedate = 2005-02-09 | df = }}</ref>

In [[statistical inference]], a [[subset]] of the population (a statistical [[sample (statistics)|sample]]) is chosen to represent the population in a statistical analysis.<ref>{{Cite web|title=Glossary of statistical terms: Sample|website=[[Statistics.com]]|url=http://www.statistics.com/glossary&term_id=281|accessdate=22 February 2016}}</ref> The ratio of the size of this statistical sample to the size of the population is called a [[sampling fraction]]. If a sample is [[Sampling (statistics)|chosen properly]], characteristics of the entire population that the sample is drawn from can be [[Estimation theory|estimated]] from corresponding characteristics of the sample{{citation needed|date=October 2017}}.

==Subpopulation==
A subconcept of a population that shares one or more additional properties is called a '''subpopulation'''. For example, if the population is all Egyptian people, a subpopulation is all Egyptian males; if the population is all pharmacies in the world, a subpopulation is all pharmacies in Egypt. By contrast, a sample is a subset of a population that is not chosen to share any additional property.

Descriptive statistics may yield different results for different subpopulations. For instance, a particular medicine may have different effects on different subpopulations, and these effects may be obscured or dismissed if such special subpopulations are not identified and examined in isolation.

Similarly, one can often estimate parameters more accurately if one separates out subpopulations: the distribution of heights among people is better modeled by considering men and women as separate subpopulations, for instance.

Populations consisting of subpopulations can be modeled by [[mixture model]]s, which combine the distributions within subpopulations into an overall population distribution. Even if subpopulations are well-modeled by given simple models, the overall population may be poorly fit by a given simple model – poor fit may be evidence for existence of subpopulations. For example, given two equal subpopulations, both normally distributed, if they have the same standard deviation and different means, the overall distribution will exhibit low [[kurtosis]] relative to a single normal distribution – the means of the subpopulations fall on the shoulders of the overall distribution. If sufficiently separated, these form a [[bimodal distribution]], otherwise it simply has a wide peak. Further, it will exhibit [[overdispersion]] relative to a single normal distribution with the given variation. Alternatively, given two subpopulations with the same mean and different standard deviations, the overall population will exhibit high kurtosis, with a sharper peak and heavier tails (and correspondingly shallower shoulders) than a single distribution

==See also==
*[[Sample (statistics)]]
*[[Sampling (statistics)]]
*[[Data collection system]]

==References==
{{Reflist}}

==External links==
*[http://www.socialresearchmethods.net/kb/sampstat.htm Statistical Terms Made Simple]

{{statistics|collection}}

[[Category:Statistical theory]]

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