Re: st: How to estimate exponential decay parameter?

 From "Song" To Subject Re: st: How to estimate exponential decay parameter? Date Sun, 23 Mar 2008 15:30:57 -0600

Thank you. That is very helpful.

Reo.

----- Original Message ----- From: "Maarten buis" <maartenbuis@yahoo.co.uk>
To: <statalist@hsphsun2.harvard.edu>
Sent: Sunday, March 23, 2008 1:03 PM
Subject: Re: st: How to estimate exponential decay parameter?

```--- 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

Buitenveldertselaan 3 (Metropolitan), room Z434

+31 20 5986715

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

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