Re: st: nl with constrains

["E. Paul Wileyto" <epw@mail.med.upenn.edu>]

An easy way to impose b>0 is to substitute b=exp(c).  c is 
unconstrained, and would have a range from minus to plus infinity, but b 
would be greated than 0.

Likewise, for k substitute k=((k2-k1)/(1+exp(m))+k1.  m is 
unconstrained, but k would fall between k1 and k2. 
I use these tricks all the time and they work very well.  To get back 
estimates and standard errors for b and k, use nlcom.

P

Olga Lyashevska wrote:

> Dear all,
>
> We have a negative exponential growth model.
> y=a exp(-kx)+b
> Is there is a way to impose constrains on model parameters such as b>0 and
> k1<k<k2?
>
>
> Thank you in advance,
> Olga
>
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--
E. Paul Wileyto, Ph.D.
Assistant Professor of Biostatistics
Tobacco Use Research Center
School of Medicine, U. of Pennsylvania
3535 Market Street, Suite 4100
Philadelphia, PA  19104-3309

215-746-7147
Fax: 215-746-7140
epw@mail.med.upenn.edu
http://mail.med.upenn.edu/~epw/ 
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