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*Subject*: Re: geometric mean?*From*: Andrew <noymer(at)my-deja.com>*Date*: Fri, 08 Sep 2000 16:59:06 GMT*Newsgroups*: comp.lang.idl-pvwave*Organization*: Deja.com - Before you buy.*References*: <8p8pni$b2v$1@nnrp1.deja.com> <8p9f67$4rq$1@nnrp1.deja.com> <39B8B345.FDF4E45D@dkrz.de>*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:21339

In article <39B8B345.FDF4E45D@dkrz.de>, Martin Schultz <martin.schultz@dkrz.de> wrote: > Hi Andrew, > > I couldn't find such a routine either so I decided to hack it > together using the algorithm you suggest but including some error > cehcking and more caution with range limits or negative values. You > can find geomean.pro on my web pages: > http://www.mpimet.mpg.de/~schultz.martin/idl/html/libmartin_schultz.html > > Cheers, > Martin > Dear Martin, Thanks!!! You include checking for negative values, which would mess up the ALOG function. Since I am taking geometric means of rates that are by definition positive, I did not think of negative numbers. There is a problem, though... Someone please correct me if this is wrong; I'm not 100% sure. The way I implemented the geometric mean was not the DEFINITION of the geomean, but rather a computational SHORTCUT. The DEFINITION goes something like: GEOMEAN(Arr)=(PROD(Arr))^(1/n), where n is the number of elements, and PROD is the product operator. Logging both sides gets rid of the nasty "nth root" (i.e. ^(1/n)) and turns the product into a sum, which is also nice. Then exponentiating un-transforms the log. Clearly we can't log any negative number, but we can product a bunch of numbers and then take an nth root of the result. And if there are zero or an even number of negative numbers there will be a real nth root, hence (I guess), the geomean would exist. I don't know what the convention is with negative numbers, and it doesn't affect me because I am using positive numbers, but maybe someone out there knows: (1) Is geomean by convention undefined if any numbers in the set are negative? (2) Is geomean always the positive nth root? geomean of -2 and -2 is +2? Cheers, Andrew Sent via Deja.com http://www.deja.com/ Before you buy.

**References**:**geometric mean?***From:*Andrew

**Re: geometric mean?***From:*Andrew

**Re: geometric mean?***From:*Martin Schultz

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