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Re: st: interpretation of squared term


From   Maarten buis <maartenbuis@yahoo.co.uk>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: interpretation of squared term
Date   Fri, 27 Aug 2010 07:13:43 +0000 (GMT)

--- At 12:39 PM 8/26/2010, David Bai wrote:
> > I include the squared term because this year variable
> > might be non-linearly related to the outcome. Is there an
> > easier way to roughly interpret the year effect? I do not
> > want to go deeper to have an exact interpretation of the two
> > year variables. E.g., what does a negative coefficient for a
> > squared term mean? 

--- On Thu, 26/8/10, Sarah Elizabeth Edgington wrote:
> The basic intuition here is that you're generating a
> quadratic curve.  If you have a positive effect of age
> and a negative effect of age squared that means that as
> people get older the effect of age is lessoned.

This may or may not be the case, that just depends on 
where the maximum of that curve is. So if you want to 
interpret just the coefficients, you need to be able to 
derive where the maximum is and what the shape of the 
curve is. It is not hard, but it requires that both you
and your readers know how to this arithmatic, which is
probably too much to ask for most readerships. In other
words, the only way to comunicate these effects is to 
graph it.

The alternative is to represent the non-linearity with a 
linear spline. Those coefficients can be interpreted 
directly without graphing.

Hope this helps,
Maarten

--------------------------
Maarten L. Buis
Institut fuer Soziologie
Universitaet Tuebingen
Wilhelmstrasse 36
72074 Tuebingen
Germany

http://www.maartenbuis.nl
--------------------------


      

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