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| From | Austin Nichols <austinnichols@gmail.com> |
| To | statalist@hsphsun2.harvard.edu |
| Subject | Re: st: non linear equation difficulties with nl function evaluation program |
| Date | Wed, 25 Aug 2010 10:45:25 -0400 |
Carolina Lopez <carolina.univ@gmail.com>: Also consider whether there are theoretical limits on parameters that -nl- could not know about, and which could lead it into infeasible areas of the parameter space; for example, if k is always positive, you might prefer to specify lnk as a parameter and then put exp(`lnk') in your formulas. On Wed, Aug 25, 2010 at 7:26 AM, Carolina Lopez <carolina.univ@gmail.com> wrote: > Thanks both for your inputs. > > I´ll review the model again to see how I can simplify and fit it, > focusing on k which is my key parameter. > > Carolina > > 2010/8/25 Nick Cox <n.j.cox@durham.ac.uk>: >> I agree with Maarten's stance here. >> >> Has anyone actually fitted such a model with a success in your area? There are plenty of fields in which complicated models are copied from paper to paper and book to book, yet no-one has ever properly tested or fitted them. >> >> There is plenty of cautionary literature, e.g. >> >> http://demonstrations.wolfram.com/FittingAnElephant/ >> >> Wei, J. 1975. Least square fitting of an elephant. CHEMTECH 5: 128–129. >> >> Nick >> n.j.cox@durham.ac.uk >> >> Maarten buis >> >> --- On Wed, 25/8/10, Carolina Lopez wrote: >>> I did start with a simple equation, which converged and >>> gave significant parameters. But when I extended it to this >>> complete version, I started finding issues. >>> >>> The thing is as alpha, k and n are in all my terms, I >>> cannot do (or do not know how) independent programs that >>> then get built up on one equation to estimate. >> >> Removing paramters is one way of simplifying an equation, >> you can also remove other parts, or change the equation in >> other ways. I cannot explain it more clearly than that you >> need to look at the equation to find a suitable >> simplification. Sounds simple, but I know it is hard and >> frustrating work. >> >> The plus of this is, is that once you are done, you'll not >> only be able to estimate your model, you will also have a >> clearer understanding of it. It is not much of comfort now, >> but it may be one in the future. >> >> * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/