<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 ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
