[Mediation-information] question about 'mediation'

[Daniel Lim]

Thanks so much for the response. To delve a little further, what I'd really
like to do is say what the total effect of the mediator is. To further
sketch out the scenario:

M = b1_treatment*T
Y = b_mediator*M + b2_treatment*T
Y = b_treatment*T
b_treatment = ATE = direct eff + indirect eff

I get all of that.  What I'm unclear about is how I can interpret the
indirect effect in conjunction with b_mediator.  Could I say something like
"the effect of the mediator is (indirect effect + b_mediator) per unit
mediator" or is the indirect effect already accounted for in b_mediator?
 Or is the situation more like:

total effect of mediator = (1-p)indirect_effect + b_mediator
b_mediator = p*indirect_effect + independent effect of mediator

In substantive terms, I'm not only interested in the effect of the
treatment but that of the mediator.

On Tue, Nov 22, 2011 at 5:58 AM, Kosuke Imai <kimai at princeton.edu> wrote:

> Hi Daniel,
>
> If you regress Y on the treatment (but not the mediator), then you will
> get the average total effect which equals the sum of direct and indirect
> effect.
>
> Kosuke
>
> Department of Politics
> Princeton University
> http://imai.princeton.edu
>
>
> On Nov 22, 2011, at 5:18 AM, Daniel Lim wrote:
>
> > Dear Professor Imai, I'm a poli sci PhD student at UCLA and I'm
> currently working on a paper involving your mediational model.  I had a
> quick question regarding the interpretation of results:
> >
> > Say that we are looking at the following linear models:
> > M = b1_treatment*T
> > Y = b_mediator*M + b2_treatment*T
> > In a nonmediational setting, we would interpret the effect of the
> mediator as b_mediator per unit M.
> >
> > When we apply "mediate" to the above, we get a result like the following:
> > Mediation Effect: X
> > Direct Effect: Y
> > Total effect X+Y
> > How do we interpret this result in conjunction with the original outcome
> model?  Is X additive with b_mediator or does it supplant that result?  In
> other words, are we saying b2_treatment=X+Y or b2_treatment+b2_mediator=X+Y
> (or are both these interpretations completely off)?
> >
> > Thanks,
> > --
> > Daniel Lim
> > PhD Student
> > Dept of Political Science, UCLA
> > daniel.k.lim at gmail.com
> > 818-480-2679 (USA)
> >
> >
>
>
>
>


-- 
Daniel Lim
PhD Student
Dept of Political Science, UCLA
daniel.k.lim at gmail.com
818-480-2679 (USA)
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