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Subject: Re: Double exponential decay fit
Date: Wed, 14 Jan 2009 18:55:03 +0000 (UTC)
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Hi Aaron,

Thanks for your reply. Yes I realized that there isn't too much of info in my first post but thanks to have reply anyway. I have made a bit of research and found that sigmaplot (analysis software) is using the Marquardt-Levenberg algorithm to find the
coefficients (parameters) of the independent variable(s) that give the &#8220;best fit&#8221;
between the equation and the data. (Shrager, R.I. (1970). Regression with Linear Constraints: An Extension of the Magnified Diagonal Method. Journal of the Association for Computing Machinery, 17, 446-452.). 

So now my question is how do I found out which algorithm Matlab is using? And how can I adapt it so it would use this algorithm?

Many thanks,
Jean-Seb


"Aaron Callard" <no.thanks@hotmail.com> wrote in message <gkl9ps$rd2$1@fred.mathworks.com>...
> "Jean-Sebastien" <newsreader@mathworks.com> wrote in message <gkl7vl$nb0$1@fred.mathworks.com>...
> > Hi folks,
> > OK :
> > You have a set of data points (time,response) which correspond to a simple two exponential decay function (peak to baseline). 
> > You feed sigmaplot10 with it and using the simple equation y=a*exp(-bx)+c*exp(-dx) it gives you a nice fit. 
> > Now you do it in Matlab using the same data, same equation and on 2 out of 3 fit you've got a funny fit which obviously is wrong... 
> > Question:
> > Does anyone can explain the decrepancy between the two softwares? why sigmaplot10 can fit a simple two exponential decay function while Matlab doesn't?
> > Is there any issue with the method used? If so is there an alternative to make Matlab fit this simple two eponential decay function correctly?
> > Thanks for your help,
> > JS
> > 
> 
> um it's hard to answer your question as you have not really given much information.  It's like me asking why C++ can give me answer but pearl can't.  Matlab is a programing language.  What algorithm are you using to fit the points, in matlab.  What is sigmaplot10?  
> 
> My guess is that the algorithm you are using in matlab is messed up slightly.  You shouldn't blame the language for code written in it.
> 
> Aaron