Curve fitting

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<ndurand <at> fr.abx.fr> writes:
Hi!

I have a problem of curve fitting.
 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!

Can you specify *exactly* what R code you're using?
Are you using nls()?

  You're trying to fit a five-parameter model to
five data points, which is likely to be difficult if
not impossible to do statistically.  Furthermore, your
data points don't have very much information in them
about all the parameters you're trying to estimate --
they are steadily decreasing, with very mild
curvature.  Finally, if these "data" happen to be
points that you have generated as theoretical
values, without adding noise, nls will give you
problems (see ?nls).

  If you give us more detail about what you're trying
to do we might be able to help (or possibly tell you
that it really can't work ...)

  Ben Bolker
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
-----------------------------------------------------------------------
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!

Nad?ge 

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