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


From   Carolina Lopez <carolina.univ@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: non linear equation difficulties with nl function evaluation program
Date   Wed, 25 Aug 2010 13:26:01 +0200

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.
>
>
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