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Newsgroups: comp.graphics.apps.gnuplot
Date: Fri, 21 Dec 2012 07:32:10 -0800 (PST)
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Subject: Re: fit gives different results for identical functions with different var names
From: Marcus Hanson <marcus.hanson@gmail.com>
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Xref: csiph.com comp.graphics.apps.gnuplot:1551

On Friday, December 21, 2012 5:42:08 AM UTC-6, Karl wrote:
> Have you initialized all your variables identically for the three
> functions? And put all the right variable names in the "via" statement? 

> Does it help to put all the variables in the function argument list? 

I have the correct 'via' statements and I have initialized the variables. Here is exactly what I have done:

gnuplot>aa=1;ma=1;sa=1;xa=1;
gnuplot>fa(xa,aa,ma,sa)=aa*exp(-(xa-ma)**2/(2*sa**2))
gnuplot>fit fa(xa,aa,ma,sa) 'test.dat' u 1:2 via aa,ma,sa

This gives aa=55,ma=0.98,sa=1 (rounded off values) with enormous standard errors (on the order of 10^16 to 10^19). These numbers do not even remotely fit the data.

On the other hand, I have tried the following:

gnuplot>a=1;m=1;s=1;x=1;
gnuplot>f(x)=a*exp(-(x-m)**2/(2*s**2))
gnuplot>fit f(x) 'test.dat' u 1:2 via a,m,s

This gives a=236,m=-1,s=-2.6 (also rounded), with standard errors less than 10%. This also fits the function remarkably well by all measures I know.

I tried various different variable names of more than one character, and they all gave exactly identical results, all of them very wrong.

Could this be some setting I accidentally changed a while ago in gnuplot?