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Re: st: How to estimate exponential decay parameter?


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: How to estimate exponential decay parameter?
Date   Sun, 23 Mar 2008 19:03:57 +0000 (GMT)

--- Song <raravise@gmail.com> wrote:
> My data shows a exponential decay pattern and I want to estimate
> exponential decay parameter. How can I estimate 'd' in the following
> model using Stata?
> 
> Sales(at t) = M*exp(-d*t)
> 
> M: initial sales
> t: time
> d: decay parameter

you can create a new variable ln(Sales) and do a normal regression of
ln(Sales) against t:

ln(Sales) = ln(M*exp(-d*t))
ln(Sales) = ln(M) + ln(exp(-d*t))
ln(Sales) = ln(M) + -d*t

The constant is ln(initial sales)
the effect of t is -d

Hope this helps,
Maarten

-----------------------------------------
Maarten L. Buis
Department of Social Research Methodology
Vrije Universiteit Amsterdam
Boelelaan 1081
1081 HV Amsterdam
The Netherlands

visiting address:
Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

http://home.fsw.vu.nl/m.buis/
-----------------------------------------


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