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Re: st: nl with constrains

From   "E. Paul Wileyto" <[email protected]>
To   [email protected]
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.


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

Thank you in advance,

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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

Fax: 215-746-7140
[email protected]
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