# Re: [R] nls not solving

From: Peter Ehlers <ehlers_at_ucalgary.ca>
Date: Tue, 01 Mar 2011 09:57:27 -0800

On 2011-03-01 06:38, Schatzi wrote:
> Here is a reply by Bart:
> Yes you're right (I should have taken off my glasses and looked closer).
> However, the argument is essentially the same:
>
> Suppose you have a solution with a,b,k,l. Then for any positive c, [a+b-bc]
> + [bc] + (bc) *exp(kl')exp(-kx) is also a solution, where l'
> = l - log(c)/k .
>
> Cheers,
> Bert
>
> (Feel free to post this correction if you like)
>
>
> This is from me:
> The problem with dropping the "l" parameter is that it is supposed to
> account for the lag component. This equation was published in the literature
> and has been being solved in SAS. When I put it in excel, it solves, but not
> very well as it comes to a different solution for each time that I change
> the starting values. As such, I'm not sure how SAS solves for it and I'm not
> sure what I should do about the equation. Maybe I should just drop the
> parameter "a." Thanks for the help.

When you say 'published in the literature' you should provide a reference; you may be misinterpreting what's published.

If SAS provides a 'solution', then there's an added assumption being made (perhaps 'l' is being fixed?). What Excel does is of little interest.

'Dropping' the parameter 'a' is equivalent to setting a=0. You could also set, say, a = -10 or l = 50, or ... The point is that, as Bert says, the model is nonidentifiable.

Peter Ehlers

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