<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

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