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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