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Re: LINFIT CHISQ and SIGMA values are correct?? [message #91598 is a reply to message #91593]

Tue, 04 August 2015 19:19

wlandsman
Messages: 743
Registered: June 2000

Senior Member

You are giving linfit negative errors -- alog10(0.2) = -0.69897

If you use the absolute value of alog10(err) you will get consistent results.

But probably it is better to do your logarithmic transformation correctly

if z = alog10(y) then dz = 0.434*dy/y (I think)

where dy is your original err and dz is your transformed err .

On Tuesday, August 4, 2015 at 3:54:39 PM UTC-4, Krishnakumar M.A wrote:
> Hi,
>
> I was trying to do a linfit in the following data (I'm using IDL 6.3).
>
> ---------------------------------------------------
>
> x = [150.0, 235.0, 325.0, 410.0, 610.0]
> y = [200.0, 35.0, 8.4, 3.0, 0.6]
> err = [25.0, 5.0, 2.1, 0.8, 0.2]
>
> result = linfit(alog10(x),alog10(y),MEASURE_ERRORS=alog10(err), CHISQ=chi, COVAR=covmatrix, SIGMA=error, YFIT=fit)

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[Message index]

		LINFIT CHISQ and SIGMA values are correct?? By: Krishnakumar M.A on Tue, 04 August 2015 12:54
		Re: LINFIT CHISQ and SIGMA values are correct?? By: wlandsman on Tue, 04 August 2015 19:19
		Re: LINFIT CHISQ and SIGMA values are correct?? By: Krishnakumar M.A on Wed, 05 August 2015 00:07
		Re: LINFIT CHISQ and SIGMA values are correct?? By: Helder Marchetto on Wed, 05 August 2015 00:23

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