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From: John Maddock (john_at_[hidden])
Date: 20071213 10:46:27
Andrew Sutton wrote:
> So I went back and read a little more carefully. Let me see if I have
> this right... Gumbel, Weibull, and FisherTippet are are all Extreme
> Value distributions. The FisherTippet distribution corresponds to a
> maximum extreme value distribution, and Gumbel, a minimum. Rereading
> the Wikipedia entry for it, they seem to use the distributions a
> little more interchangeably.
I'm not sure that's correct still and different sources use different
conventions.
Mathworld seems to treat the terms Extreme Value and Fisher Tippet as
interchangable (see
http://mathworld.wolfram.com/ExtremeValueDistribution.html), but if someone
refers colloquially to "The Extreme Value" or "The Fisher Tippet" or "The
LogWeibul" distribution then it probably means the maximum case of the type
I extreme value dist.
NIST uses "Gumbel Distribution" to refer to both the Extreme Value Type I
distibutions (min ans max cases). Other sources refer to only the minimum
case as "The Gumbel Distribution". Mathworld refers to these as "Gumbel
types" http://mathworld.wolfram.com/GumbelDistribution.html. While
mathematica uses Gumbel to refer to just the minimum case.
So you can call it whatever you like and still be right :)
BTW I believe the min and max cases are basically just mirror images of each
other about the location parameter?
> As far as code goes, Boost.Math has the FisherTippet variant (as
> extreme_value), but not a true Gumbel distribution, right? That means
> that I've effectively written a FisherTippet variate, but not really
> a true Gumbel variate... I guess I'll just hack away at my new Gumbel
> distribution and rename the number generator.
Well it's a kind of Gumbel distribution, but not what folks normally call
"The Gumbel Distribution": which is the minimum case.
Hope that's now slightly clearer than the proverbial mud ;)
John.
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