[R] Curve fitting

ndurand@fr.abx.fr
Fri, 13 Jan 2006 08:17:19 -0000

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

----- Réacheminé par Nadege ND Durand/RD/abx/FR le 01/13/2006 09:15 -----


Nadege ND Durand
01/12/2006 19:11


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

Hi!

I have a problem of curve fitting.

I use the following data :

 - vector of predictor data :
0
0.4
0.8
1.2
1.6

- vector of response data :
0.81954
0.64592
0.51247
0.42831
0.35371

 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!

Nadège


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