Re: st: state trends

[Maarten buis <maartenbuis@yahoo.co.uk>]

--- On Tue, 19/10/10, sara borelli wrote:
> Is that correct to say that in any case the coefficient of
> my variables of interest will always be the same regardless
> of whether I use year=1990,1991  or its scaled
> counterpart? Just the coefficient on state fixed effects
> change and the units in which the trend is measured should
> not matter...is that correct?

Depends, as long as there are no interactions with time the 
unit should not matter, though I would opt for some reasonable
scale and origin for your research, which is often not years 
since the year 0.
 
> I am not sure I understand the reply to the second question
> (quadratic trend). If the quadratic trend based on orginal
> year variable causes a drop in state fixed effects  why
> it does not happen the same when I use the quadratic trend
> based on the re-scaled variable? Would be  correct at
> this point to use the quadratic trend based only on the
> re-scaled year variable?

I can think of two reasons: First, you added your quadratic terms
as a variable list var1-vark, and there could be other variables
in that list than you intend. That was my first hunch. 
Second, you may have ended up with precision and multicolinearity
problems by squaring years that range between 1990-...., while 
you to a large extend reduce those by sqaring variable ranging 
1, ...
 
As I said in my previous post: The best strategy for building 
such models, is not to do everthing at once, but instead to break
the problem up into manageable parts. In your case I would solve
the trend issue for one state, and once you got that working, try 
to extend this to your larger model by adding the interactions.

Hope this helps,
Maarten

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

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


      

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/