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MathGroup Archive 2011

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Extra parameter for DirichletDistribution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg121912] Extra parameter for DirichletDistribution
  • From: Todd <johnson.todd.r at gmail.com>
  • Date: Thu, 6 Oct 2011 04:24:18 -0400 (EDT)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Can anyone tell me why DirichletDistribution in Mathematica 8 requires
k+1 parameters for a k-dimensional distribution and why the extra
parameter cannot be 0, which would give results similar to other
software packages that require only k parameters. The function of the
extra parameter is clear from the mean of the 2D distribution given in
the help file:

Namely, the mean of DirichletDistribution[a1, a2, a3] is {a1/
(a1+a2+a3), {a2/(a1+a2+a3)}.

In this case, setting a3 to 0 would give me the desired result, but
the function requires a3 to be positive.

I noticed that a definition of DirichletDistribution from an older
Wolfram tutorial required only k parameters for a k-dimensional
distribution.

Todd



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