Isaac Roberts

2014-01-29T23:49:59Z

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In [[population genetics]], '''Ewens' sampling formula''', describes the probabilities associated with counts of how many different [[allele]]s are observed a given number of times in the sample.

==Definition==
Ewens' sampling formula, introduced by [[Warren Ewens]], states that under certain conditions (specified below), if a random sample of ''n'' [[gamete]]s is taken from a population and classified according to the [[gene]] at a particular [[locus (genetics)|locus]] then the [[probability]] that there are ''a''1 [[allele]]s represented once in the sample, and ''a''2 alleles represented twice, and so on, is

:<math>\operatorname{Pr}(a_1,\dots,a_n; \theta)={n! \over \theta(\theta+1)\cdots(\theta+n-1)}\prod_{j=1}^n{\theta^{a_j} \over j^{a_j} a_j!},</math>

for some positive number ''θ'', whenever ''a''1, ..., ''a''''n'' is a sequence of nonnegative integers such that

:<math>a_1+2a_2+3a_3+\cdots+na_n=n.\,</math>

The phrase "under certain conditions", used above, must of course be made precise. The assumptions are:
* The sample size ''n'' is small by comparison to the size of the whole population; and
* The population is in statistical equilibrium under [[mutation]] and [[genetic drift]] and the role of selection at the locus in question is negligible; and
* Every mutant allele is novel. (See also [[idealised population]].)

This is a [[probability distribution]] on the set of all [[integer partition|partitions of the integer]] ''n''. Among probabilists and statisticians it is often called the '''multivariate Ewens distribution'''.

== Mathematical properties ==

When ''θ'' = 0, the probability is 1 that all ''n'' genes are the same. When ''θ'' = 1, then the distribution is precisely that of the integer partition induced by a uniformly distributed [[random permutation]]. As ''θ'' → ∞, the probability that no two of the ''n'' genes are the same approaches 1.

This family of probability distributions enjoys the property that if after the sample of ''n'' is taken, ''m'' of the ''n'' gametes are chosen without replacement, then the resulting probability distribution on the set of all partitions of the smaller integer ''m'' is just what the formula above would give if ''m'' were put in place of ''n''.

The Ewens distribution arises naturally from the [[Chinese restaurant process]].

==See also==
* [[Coalescent theory]]
* [[Unified neutral theory of biodiversity]]

{{inline|date=August 2011}}
==Notes==
* Warren Ewens, "The sampling theory of selectively neutral alleles", ''Theoretical Population Biology'', volume 3, pages 87–112, 1972.
* J.F.C. Kingman, "Random partitions in population genetics", ''Proceedings of the Royal Society of London, Series B, Mathematical and Physical Sciences'', volume 361, number 1704, 1978.
* S. Tavare and W. J. Ewens, [http://www.cmb.usc.edu/people/stavare/STpapers-pdf/TE97.pdf "The Multivariate Ewens distribution"]. (1997, Chapter 41 from the reference below).
* N.L. Johnson, S. Kotz, and N. Balakrishnan (1997) ''Discrete Multivariate Distributions'', Wiley. ISBN 0-471-12844-9.

{{ProbDistributions|multivariate}}

{{DEFAULTSORT:Ewens's Sampling Formula}}
[[Category:Probability distributions]]
[[Category:Population genetics]]
[[Category:Discrete distributions]]

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Isaac Roberts