Date: Mon, 15 Nov 1999 16:41:44 +0000
Reply-To: John Whittington <John.W@MEDISCIENCE.CO.UK>
Sender: "SAS(r) Discussion" <SAS-L@LISTSERV.UGA.EDU>
From: John Whittington <John.W@MEDISCIENCE.CO.UK>
Subject: Re: Statistic
In-Reply-To: <199911151436.OAA22628@vicar.netnames.net>
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At 16:20 15/11/99 +0200, dwall@QBLM.QUINTILES.COM wrote:
>Has anyone heard of a geometric CV (co-efficient of variation).
>I know what a geometric mean and SD are.
>I have some test data and it appears that the geometric CV is
>the SD of log-transformed data (without anti-logging again).
>This is then multiplied by 100 to convert to a percentage.
>Is this correct???
I can't say I've heard of a 'geometric CV' but I would be rather surprised
if it were what you have described. By analogy with a normal 'CV', one
would imagine that a geometric one would have to be expressing the
variability in terms of the mean in some fashion - whereas what you
describe is simply a function of the SD.
If you had said that it was the SD of log-transformed data DIVIDED BY the
mean of log-transformed data (both without anti-logging), then multiplied
by 100, then that would have made much more sense.
Another possibility, of course, would be to take it as SD/mean*100, using
the back-transformed (i.e. anti-logged) SD and mean - perhaps reasonable in
some senses, in as much as it is only really after back-transformation that
the SD and mean *are* really 'geometric'. Otherwise, what you'd be talking
about would be 'the CV of log-transformed data'.
I'm sure someone around will know the true/conventional answer!
Regards
John
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