Curve fitting

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Curve fitting

ndurand
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 :  [hidden email]
        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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Re: Curve fitting

Ben Bolker-2
 <ndurand <at> fr.abx.fr> writes:

>
> 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 -----
>

  it doesn't matter; all the basic issues raised by me and
by Albyn Jones still apply.  If we are going to help you,
you need to provide more detail about what you tried in R
and more background on what you're trying to achieve in general.
As we both said, fitting a 5-parameter model to 5 data
points is nearly impossible.  If you provide more detail
about your data and your broader goal, we might be able
to help.

  Ben Bolker

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