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From: "John D'Errico" <woodchips@rochester.rr.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: exponential fitting?
Date: Fri, 26 Sep 2008 17:42:01 +0000 (UTC)
Organization: John D'Errico (1-3LEW5R)
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"Mario " <nospam@yahoo.com> wrote in message <gbj5ao$1rr$1@fred.mathworks.com>...
> Hi..
> I'm dealing with this kind of data
>  
> x=[527.3756  546.1192  564.9728  586.4216  608.2959  630.5845  652.8730  674.8214  697.0359]
> 
> y=[193  204.1192  214.9728  219  222  224 222 219.1786  217]
> 
> and i would like to fit a*(1-exp(b*x))   (because i think it's the best curve that fits these points) and i tried to do it but i didn't get a good fit! What am i doing wrong?
> I even tried to scale the data in order to place the first point in [0,0] but it didn't work as i expected (the exponential curve fit the points after it grows, so it fits the point with a line almost)
> 
> Hope you can help me! 

That might be the best curve to fit your
class of data, but that data does not fit
your choice of model even slightly well.

Note that it rolls over, something an
exponential cannot do.

Worse, suppose that you did find a set
of coefficients for that model. The fact
that it MUST pass through (0,0) forces
the shape of the curve to not fit this data.

Sorry, but this is a TERRIBLE choice of
model for your data. That is reflected
in the poor resulting fits that you find.

John