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Re: st: nl with constrains


From   "E. Paul Wileyto" <epw@mail.med.upenn.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: nl with constrains
Date   Fri, 09 May 2008 07:00:21 -0400

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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