[Mediation-information] Dear Dr. Imai:

[Kosuke Imai]

Dear Glenn,

   First, let me point you towards a few papers we've written on this 
topic.   The software is designed to implement the methods proposed in our 
paper: http://imai.princeton.edu/projects/mechanisms.html

   The easiest paper to read is the working paper titled "Unpacking..." 
though this one is targeted to political scientists.  Given your 
background, the Psychological Methods paper may be the one you might want 
to read first.

   Next, we have just released a new version 3.0 to CRAN with lots of 
additional functionalities.  We also have a mailing list where you can 
post questions and get some information about the updates, etc.

http://r-forge.r-project.org/projects/mediation/

   Finally, Please see below my answers to your question:

On Mon, 2 May 2011, Glenn Walters wrote:

> Dear Dr. Imai:
>
> I just started using your causal mediation program for R and had a few
> questions.

  Have a look at this vignette, which is an updated version of our Springer 
chapter: 
http://cran.r-project.org/web/packages/mediation/vignettes/mediation.pdf 
This details the software and its use with examples.

> 1. I noticed that the independent variable in all of your examples was
> some form of treatment in which participants were randomly assigned to
> conditions.  Is it legitimate to use your program with a non-manipulated
> independent variable like history of mental health problems?

  Yes in principle but of course in observational studies it's important to 
adjust for pre-treatment variables so that the treatment and control 
groups are comparable.

> 2. What do the output values for the mediation effect, direct effect,
> and total effect represent?  I am thinking that this represents amount
> of variance explained ( R-square) but I wanted to make sure.

Please have a look at our Unpacking or Psychological Methods papers.  It's 
not really about R^2.

> 3. How do I interpret the sensitivity results?  The two R-square values
> at the bottom of the output window seem to make the most sense, but does
> a low value (e.g., .0033) indicate good or poor robustness to violations
> of key identifying assumptions, or does this depend on the absolute
> level of the mediation effect?

Again, please have a look at the Psych Methods paper and the software 
vignette.

> Thanks for any light you might be able to shed on these issues.
>
> Glenn D. Walters, Ph.D.
> Psychology Services
> FCI-Schuylkill
> Minersville, PA  17954
>
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