RE: st: non linear equation difficulties with nl function evaluation program

[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.


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