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Topic: Trouble using polyfit that separated straight fit from original
data points

Replies: 1   Last Post: Nov 4, 2013 7:59 AM

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Torsten Hennig

Posts: 2,419
Registered: 12/6/04
Re: Trouble using polyfit that separated straight fit from original
data points

Posted: Nov 4, 2013 7:59 AM
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> Hi all,
> Thought I might post here too.
> I am plotting some data from a power-inverse law and
> using the following:
> loglog(E2dist,E2PrdB,'+g'), hold on
> p = polyfit(log(E2dist),log(E2PrdB),1);
> m = p(1);
> b = exp(p(2));
> %mean=mu(1);
> %std=mu(2);
> loglog(E2dist, b.*E2dist.^m,'-m*');
> This works fine. But when I change the polyfit line
> to
> [p,S,mu] = polyfit(log(E2dist),log(E2PrdB),1);
> so that I can get standard deviation etc. my straight
> line fit is moved vertically up from my original data
> points? I notice that the log of my first dist point
> 1m is coverted to 0. Not sure if that would move it
> and how to correct it.
> Hope you can help.
> cheers

Did you notice that when you call polyfit as
[p,S,mu] = polyfit(log(E2dist),log(E2PrdB),1);
the resulting regression polynomial will be a function of
x^= (x-mu)/sigma instead of x ?

Best wishes

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