From: Albyn Jones <jones_at_reed.edu>

Date: Fri 13 Jan 2006 - 06:54:38 EST

On Thu, Jan 12, 2006 at 07:11:12PM +0100, ndurand@fr.abx.fr wrote:

*> Hi!
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*>
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> I have a problem of curve fitting.

*>
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*> I use the following data :
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*>
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*> - vector of predictor data :
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*> 0
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*> 0.4
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*> 0.8
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*> 1.2
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*> 1.6
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*>
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*> - vector of response data :
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*> 0.81954
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*> 0.64592
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*> 0.51247
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*> 0.42831
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*> 0.35371
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*>
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*> I perform parametric fits using custom equations
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*>
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*> when I use this equation : y = yo + K *(1/(1+exp(-(a+b*ln(x))))) the
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*> fitting result is OK
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*> but when I use this more general equation : y = yo + K
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*> *(1/(1+exp(-(a+b*log(x)+c*x)))) , then I get an aberrant curve!
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*>
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*> I don't understand that... The second fitting should be at least as good
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*> as the first one because when taking c=0, both equations are identical!
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*>
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*> There is here a mathematical phenomenon that I don't understand!....could
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*> someone help me????
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*>
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*> Thanks a lot in advance!
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*>
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*> Nad?ge
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*>
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*> [[alternative HTML version deleted]]
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*>
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R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Fri Jan 13 07:06:07 2006

Date: Fri 13 Jan 2006 - 06:54:38 EST

You haven't told us how you are fitting the model; are you using nls(), and if so with what initial values? The models don't make sense at x=0, due to the inclusion of the log(x) term. Ignoring that, you have 5 observations and 5 parameters in your second model. What is the reason you are including both "b*log(x)" and "c*x" terms in the model?

regards

albyn

On Thu, Jan 12, 2006 at 07:11:12PM +0100, ndurand@fr.abx.fr wrote:

> I have a problem of curve fitting.

*> ______________________________________________
*

> R-help@stat.math.ethz.ch mailing list

*> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
*

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Fri Jan 13 07:06:07 2006

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