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Re: st: question about dummies for time periods


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: question about dummies for time periods
Date   Wed, 16 Jul 2008 10:22:22 +0100 (BST)

That is not a problem. By turning time into dummies, the interpretation
is no longer "if time changes by a decade, y changes ... units", but
"the expected value of y in period ... differs ... units from expected
y in the reference period". So the unit of time is no longer part of
the interpretation. Another way to see that is that you now assume that
the effect of time on the dependent variable follows a step function:
the expected value of y is constant during the first period, than
changes suddenly between the first and the second period, remains
constant during the second period, than changes suddenly between the
second and the third period, etc.

-- Maarten

--- Carola Herrera <herrera.carola@yahoo.com> wrote:

> > Dear all,
> > 
> > Is it right to use dummies for periods of time of different
> > length in the same regression?  I have one dummy for a
> > decade, another dummy for a second decade, but the last
> > dummy is for only 4 years.  Is this right for STATA or should all
> the
> > dummies used in the same regression be even in terms of # of
> > years?  The baseline is a decade.
> > 
> > thansk in advance
> Regards,
> > Carola Herrera
> 
> 
> 
>       
> 
> *
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> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
> 


-----------------------------------------
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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