
From:  Doug Stewart 
Subject:  Re: Stepfunction fit using leasqr 
Date:  Thu, 1 Sep 2016 16:03:31 0400 
Dear all,
I am trying to fit measured data using a step function, please see the attachments. I get an error "weighted residuals are not real". Ok, additionally to a conceptual glitch I might have bad initial values but I thought Marquardt Levenberg was supposed to converge even in that case. What am I missing?
I appreciate any suggestions. I might be on the wrong track with the model function and the initial values, so if there are any better approaches please let me know.
Why do I want to fit this data? Firstly, I need some kind of lookup for various xvalues which might not coincide with measured xvalues. Secondly, I need to get rid of the scatter in the end of the measurement for further processing. Is there perhaps a simpler way to achieve this?
Code for reference w/o attachment:
E = dlmread('data.txt');
x = E(:,1);
y = E(:,2);
q = [1; .25; .25];
function res = f(x,q)
res = q(1)  exp((q(2).*x).^q(3));
end
[fr,q,cvg,iter_r] = leasqr(x,y,q,'f',.0001,40);
Any help would be appreciated. Thanks and kind regards, Jan
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