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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