[Mediation-information] moderated mediation analysis using 'mediation' package

[Teppei Yamamoto]

Hi Liviu,

Glad that you got it to work and thanks for your feedback. We'll consider that for future updates. For now though, people can use your code as a template (and we can refer to it when asked). 

Best, 
Teppei

=========================
Teppei Yamamoto
Assistant Professor
Department of Political Science
MIT
teppei at mit.edu
=========================


On Mar 23, 2012, at 7:10, Liviu Andronic <landronimirc at gmail.com> wrote:

> Hello
> 
> 
> On Thu, Mar 22, 2012 at 7:47 PM, Teppei Yamamoto <teppei at mit.edu> wrote:
>> One thing you could do is to set the moderator to different values along its
>> range (running a separate mediate call for each of those values) and plot
>> the estimated mediation effects against those moderator values. That way,
>> you will see how the indirect effect (or direct effect if that's what you
>> are more interested in) varies as a function of the moderator, which I think
>> is exactly the goal of moderated mediation analysis.
>> 
> Thank you again for all the explanations.
> 
> I managed to estimate the mediation effect over the range of the
> mediator (although it's quite CPU-consuming)
> #estimate mediation effects
> med_c <- lapply(quantile(modmed$ENGAGESP), function(x){
>    mediate(mod.m2, mod.y2, dropobs=F, treat='EVPUBLIC', mediator='TRUST',
>    covariates=list(ENGAGESP=x),  boot=T, sims=50)
> })
> 
> 
> and then plotting the resulting objects
> #plot mediation effects
> par(mfrow=c(2,3))
> lapply(1:length(med_c), function(x) plot(med_c[[x]], main=names(med_c[x])))
> par(mfrow=c(1,1))
> 
> 
> and subsequently plot the estimated mediation effects against the
> moderator values.
> #plot the estimated mediation effects against the moderator values
> (xa <- ldply(1:length(med_c), function(x)
> data.frame(med_c[[x]][c('d0', 'z0')])))
> plot(quantile(modmed$ENGAGESP), xa[,'d0'])
> plot(quantile(modmed$ENGAGESP), xa[,'z0'])
> 
> 
> This wasn't immediately straightforward, however, and required some
> vectorizing magic. Could 'mediation' provide some functions or methods
> that would help automating some of the procedures above?
> 
> Regards
> Liviu