[R] Curve fitting

From: <ndurand_at_fr.abx.fr>
Date: Fri 13 Jan 2006 - 19:17:19 EST

I made a mistake in my equations : all the logarithms are neperian!

Nadege ND Durand
01/12/2006 19:11  

        Pour :  r-help@lists.R-project.org
        cc : 
        Objet : Curve fitting 


I have a problem of curve fitting.

I use the following data :

 I perform parametric fits using custom equations

when I use this equation : y = yo + K *(1/(1+exp(-(a+b*ln(x))))) the fitting result is OK
but when I use this more general equation : y = yo + K *(1/(1+exp(-(a+b*log(x)+c*x)))) , then I get an aberrant curve!

I don't understand that... The second fitting should be at least as good as the first one because when taking c=0, both equations are identical!

There is here a mathematical phenomenon that I don't understand!....could someone help me????

Thanks a lot in advance!


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