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Subject: Re: Important Regression Questions
Date: Wed, 4 Nov 2009 18:54:02 +0000 (UTC)
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"Samoline1 Linke" <maganatewoman@yahoo.com> wrote in message <hcscmu$8u9$1@fred.mathworks.com>...
> Hi,
> 
> I have a few confusions about Regression!
> 
> First, I have seen regression can be applied to either 'linear' or 'non linear' models 
> 
> BUT!!!

I'm not an expert, but my experiences are:

> 
> 1 - What is the best criteria to see that the fitted curve or line is the best model e.g. one way could be to compare the values of norm of residuals....is there any other? 

Yes, there are many others, but no "best" criteria other than plotting and using your own eyeball to judge which is best given YOUR criteria.
> 
> 2- when I do regstats (x,y), then it gives me very useful information e.g. value of   R-square etc. but are these values calculated based on a linear model or does it calculate the values fitting the best possible model?

I believe it calculates the values based on the model YOU supplied.
(There is no a-priori "best possible model" under ALL circumstances.

> 
> 3- why is it so that for nonlinear, one must have some formula in mind to apply. Is not it possible that I simply input my data and matlab itself decides whats the best fit (non linear).

No, I don't believe it is possible.
There are an infinite number of nonlinear models that can be applied.
How can MATLAB try all the infinite possibilities?

> 
> Thanks for ur time.