You could try fitting several cases: - fit the reduced model with only one exp term, i.e. one of the alphas is 0 - fit the model with both alpha's constrained to be positive & sufficiently away from 0 - fit the model with both alpha's negative & sufficiently away from 0 - fit the model with one positive and one negative and sufficiently away from 0
On 9/18/06, Sebastian P. Luque <[EMAIL PROTECTED]> wrote: > On 18 Sep 2006 17:55:24 +0200, > Peter Dalgaard <[EMAIL PROTECTED]> wrote: > > > "Gabor Grothendieck" <[EMAIL PROTECTED]> writes: > >> Here are some approaches: > > >> - we only have 4 parameters so just use grid search to get starting > >> values as in: > > >> https://stat.ethz.ch/pipermail/r-help/2005-September/079617.html > > >> - there are singularities near beta_1 = beta_2 and near alpha_1 = 0 and > >> near alpha_2 = 0 so reparameterize and use the upper and lower bounds > >> to avoid those regions. You could try a separate reduced model for > >> those. > > > > Or just use SSbiexp and reparametrize (it is exactly the same model) > > Thank you Gabor and Peter. With your help, I was able to fit the model > using SSbiexp as Peter suggested. However, the dependent variable spans > both negative and positive values, and the negative values cause the > fitting procedure to fail with a NA/NaN/Inf error. This might be related > to the singularities that Gabor mentioned. If the dependent variable is > shifted upwards, so all values are positive, the model is fit without > problems. Any ideas on how to get around this issue without shifting the > data, so that the parameters are expressed in the original scale? > Hopefully it's possible to do it using SSbiexp. Thanks again. > > > Cheers, > > -- > Seb > > ______________________________________________ > [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 > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ [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 and provide commented, minimal, self-contained, reproducible code.
