[Mediation-information] Appropriate use of package "mediation"

Kosuke Imai kimai at Princeton.EDU
Thu Sep 15 04:19:22 CEST 2011

Hi Tormod,

  I'm ccing the listserv we have so that my collaborators and others can give additional insights.  The mediation software doesn't have the functionality to fully accomodate the continuous treatment.  However, when the software will calculate the average causal mediation effect of changing the treatment from 0 to 1.  If you use a linear model for both mediator and outcome, then this should be sufficient because the ACME would not change regardless of the base value you choose for the treatment.  If you have a nonlinear model, however, the ACME you get is still valid but only for the scenario where you change the treatment value from 0 to 1.  If you are thinking about different scenario (e.g., changing the treatment from 3 to 4), then you will get a different answer.  


Department of Politics
Princeton University

On Sep 14, 2011, at 8:24 AM, Tormod Bøe wrote:

> Dear Associate Professor Imai,
> I am currently doing a PhD at the University of Bergen. We have conducted a large-scale population based study of childhood mental health.
> I am in the process of writing at paper where we investigate to which extent sleep problems mediate the association between parental socioeconomic status and children's mental health problems.
> We have two sleep measures (one rated on a three point scale, and two that are binary[0=adequate sleep time, 1=long/short sleep time]), parental SES is continuous (years of education and family income rated on five point scales) and outcome is continuous (range 0-30).
> I have made some attempts at running mediation analyses using your R package mediation. I have made one model m where I regress family SES on sleep problems, and model y where I regress family SES and sleep problems on mental health problems outcome. I use sleep problems as "mediator" and have used SES variables (e.g. family economy) as treatment variables. However, is this an appropriate use of your package?
> I am especially concerned with regards to the use of a continuous "treatment" variable. The models run without any errors, and the pattern of results (with regards to significant interactions) resemble those I obtain when running path analyses in Mplus.
> If you could please provide some advice in this manner it would be greatly appreciated.
> Yours sincerely,
> Tormod Bøe

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