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

[Michael McCulloch <mm@pinest.org>]

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 <mm@pinest.org> 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/
> -----------------------------------------
>
>
>
>
>
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Best wishes,
Michael


____________________________________

Michael McCulloch
Pine Street Clinic
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