

ガンマ分布は、ベータ分布に関連する一般的なタイプの統計分布であり、ポアソン分布イベント間の待ち時間が関連 ガンマ分布には、と
というラベルの二つの自由パラメータがあります。,c0a5aa4824″>





for , where
is a complete gamma function, and
an incomplete gamma function.,
整数の場合、この分布はErlang分布と呼ばれる特殊なケースです。,02b6″>





Now let (not necessarily an integer) and define
to be the time between changes., Then the above equation can be written
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(13)
|
for . This is the probability function for the gamma distribution, and the corresponding distribution function is
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(14)
|
where is a regularized gamma function.,
これはWolfram言語でGammaDistribution関数として実装されています.,id=”c43570dc99″>




giving moments about 0 of
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(19)
|
(Papoulis 1984, p., 147).,iv>






The gamma distribution is closely related to other statistical distributions., If ,
, …,
are independent random variates with a gamma distribution having parameters
,
, …,/div>



Also, if and
are independent random variates with a gamma distribution having parameters
and
, then
is a beta distribution variate with parameters
., いずれも以下のように導出できる。,






where is the beta function, which is a beta distribution.,
If and
are gamma variates with parameters
and
, the
is a variate with a beta prime distribution with parameters
and
.,iv>



The ratio therefore has the distribution
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(50)
|
which is a beta prime distribution with parameters .,






where is the Pochhammer symbol.,0822e6ea8″>
so the cumulants are
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(63)
|
If is a normal variate with mean
and standard deviation
, then
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(64)
|
is a standard gamma variate with parameter .,