[Mediation-information] question about 'mediation'

[Kosuke Imai]

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)
> 
>