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Re: routine for geometric mean regression [message #28368 is a reply to message #28278]

Sun, 02 December 2001 21:47

Wayne Landsman
Messages: 117
Registered: January 1997

Senior Member

John Miller wrote:

> Does anyone know of or have a geometric mean regression routine?
>
> I have the routine 'fitexy.pro' and am aware of the IDL version of
> ODRpack. These do what I need (fit a line with errors in y AND x), but
> I'd like to compare these with the geometric mean approach.

If I understand what you mean by "geometric mean" correctly, then the
following 3 lines of IDL should do it. I assume you have data (X,Y)
with corresponding error bars SIGX and SIGY

resulty = linfit(x,y,sdev = sigy) ;Fit Y vs. X using Y error
bars
resultx = linfit(y,x,sdev = sigx) ;Fit X vs. Y using X error
bars
slope = sqrt(resulty[1]/resultx[1]) ;Take geometric mean of two slope
determinations

But, as you note, the correct way to fit a straight line with errors in
both coordinates is given by the procedure
http://idlastro.gsfc.nasa.gov/ftp/pro/math/fitexy.pro

Wayne Landsman landsman@mpb.gsfc.nasa.gov

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[Message index]

		routine for geometric mean regression By: john.b.miller on Fri, 30 November 2001 13:40
		Re: routine for geometric mean regression By: Wayne Landsman on Sun, 02 December 2001 21:47

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