Statalist The Stata Listserver


[Date Prev][Date Next][Thread Prev][Thread Next][Date index][Thread index]

Re: st: swaic, different models achieved with forward vs. backward


From   Michael McCulloch <[email protected]>
To   [email protected]
Subject   Re: st: swaic, different models achieved with forward vs. backward
Date   Thu, 14 Dec 2006 17:53:45 -0800

Maarten and Nick,
Thank you both for your astute comments. They are very helpful.
Michael

At 02:24 PM 12/14/2006, you wrote:
--- Michael McCulloch <[email protected]> wrote:
> I'm comparing forward vs. backward options for swaic, after stcox.
> Different models are achieved. Is there a rule-of-thumb about which
> to use?

Don't add two variables that measure the same thing, that way you are
controlling a variable for itself.

Do control for a variable that causes both the explanatory variable of
interest and the dependent variable. Classic example is the quality of
the summer in the previous year in a rural area causes both the number
of storks to increase (more food, more baby storks survive) and causes
the number of humans babies to increase (good harvest, more positive
outlook on the future, and don't forget the nice harvest party). If you
don't control for the weather during previous summer you might think
that the number of storks determine the number of babies.

If two explanatory variables are just correlated, then it depends
whether you want the total effect, i.e. the direct effect plus indirect
effect, or the direct effect. The total effect usually makes more
descriptive sense to me, but the direct effect is closer to a causal
story.

Which scenario fits which variable in your model is determined by your
substantive knowledge of the problem you are investigating.

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/
-----------------------------------------





___________________________________________________________
All new Yahoo! Mail "The new Interface is stunning in its simplicity and ease of use." - PC Magazine
http://uk.docs.yahoo.com/nowyoucan.html
*
* For searches and help try:
* http://www.stata.com/support/faqs/res/findit.html
* http://www.stata.com/support/statalist/faq
* http://www.ats.ucla.edu/stat/stata/

Best wishes,
Michael


____________________________________

Michael McCulloch
Pine Street Clinic
Pine Street Foundation
124 Pine Street, San Anselmo, CA 94960-2674
tel     415.407.1357
fax     415.485.1065
email:  [email protected]
web:    www.pinest.org
        www.pinestreetfoundation.org
        www.medepi.net/meta





*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/



© Copyright 1996–2024 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index