[R-sig-ME] Mixed models and mediation

[Jonathan Baron]

Adam,

The following article might help:

MacKinnon, D. P., Lockwood, C. M., Hoffman, J. M., West, S. G., &
Sheets, V. (2002).  A comparison of methods to test mediation and
other intervening variable effects.  Psychological Methods, 7, 83-104.

It argues that you can test mediation very simply by taking the
maximum of two p-values.  One is the regression of the mediator M on
the independent variable X.  The other is the coefficient for M when
the dependent variable Y is regressed on M and X.  This seems slightly
better than the Sobel test, and it may avoid the need for
bootstrapping.  (I have also run some simulations, and, indeed, this
method is very good.)

Another article by Lois Gelfand (2008?, maybe 2009) in "Journal of
General Psychology" reviews the more recent literature but does not (I
think) change the main conclusion.

So far as I can tell, the use of mixed models should not change these
conclusions.  You still have to get p-values, though.

Jon

On 11/30/09 22:30, Adam D. I. Kramer wrote:
> Hello,
> 
>  	Could anyone recommend a document or resource for doing a mediation
> analysis for some glmer models?  I've seen a few hints of "mediation using
> mixed models" in general online (something akin to "do a sobel test with the
> estimates and standard errors, but bootstrap significance"), but no examples
> of anybody doing this in R.
> 
>  	My research question is basically summarized like this: Does whether
> a person (subjID) chooses an option offered to them (chosen) depend on the
> value of that option (value) as well as how many options they've seen
> already (option)?  Specifically, does adding "value" to the model partially
> mediate the role that option plays?  There is also another nesting factor, a
> between-subjects condition (thisDist), in which values are nested.
> 
> g <- glmer(chosen ~ option + value + (1|subjID) + (value|thisDist), data=r1,
> family="binomial")
> 
> ...my intuition would be to use boot() to randomly vary the levels of
> "value" within each subject and re-run glmer() a few thousand times to
> estimate a standard error for the fixed effect of "option" with something
> like "value" in the model, but I wanted to see whether anybody had done an
> analysis like this before I think too hard about reinventing the wheel.
> 
> Many thanks,
> --
> Adam D. I. Kramer
> Ph.D. Candidate, Social Psychology
> University of Oregon
> adik at uoregon.edu
> 
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-- 
Jonathan Baron, Professor of Psychology, University of Pennsylvania
Home page: http://www.sas.upenn.edu/~baron
Editor: Judgment and Decision Making (http://journal.sjdm.org)