[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: nl with constrains

From   "E. Paul Wileyto" <>
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,

* For searches and help try:

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
* For searches and help try:

© Copyright 1996–2015 StataCorp LP   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index