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.
