[Mediation-information] Question about results from R mediation package

[Alexander Weiss]

Dear Teppei and Kosuke,

Thank you for clarifying that and for catching my error. If I have any further 
questions, I will let you know.

Best,

Alex

On Thursday 12 Apr 2012 05:28:47 Teppei Yamamoto wrote:
> Hi Alex,
> 
> Kosuke is right about the scale of the output. That is, the effect 
> represents the average difference in the time until death between the 
> treatment and control conditions, decomposed into indirect and direct 
> components.
> 
> Looking at your code, I think you are not correctly using the 
> "covaritates" argument in the mediate function --- you don't have to 
> write out those variable names unless you want to make inferences 
> conditional on specific values of those covariates. This is sometimes 
> called the "moderated mediation" analysis by psychologists. See the help 
> file, especially the example section.
> 
> In your message, you say at the beginning that your treatment is 
> continuous but then you ask a question about a categorical treatment 
> variable. Regardless, you can specify a specific contrast using the 
> "treat.value" and "control.value" arguments. Again please refer to the 
> help file.
> 
> Best,
> Teppei
> 
> Kosuke Imai wrote:
> > Hi Alex,
> >
> >    The scale of the quantities of interest (the average causal mediation 
effects and average direct effects) is the same as that of the outcome variable; 
that is, the time to death in the survival analysis.  I'm ccing my 
collaborators through our mailing list so that they can confirm: see https://r-
forge.r-project.org/mail/?group_id=1070
> >
> > Best,
> > Kosuke
> >
> > Department of Politics
> > Princeton University
> > http://imai.princeton.edu
> >
> > On Apr 11, 2012, at 9:33 AM, Alexander Weiss wrote:
> >
> >> Dear Prof. Imai:
> >>
> >> I have recently been asked to determine whether the effect of cigarette 
smoking
> >> (non = 0, former = 1, present = 2) mediates the effect of a personality 
trait
> >> (Agreeableness, a continuous variable scaled as a z-score) on time to 
death in
> >> an accelerated failure time model, i.e., a model of the type 
survreg(Surv(y))
> >> with a Weibull distribution in R.
> >>
> >> I used the package developed by yourself and your colleagues, though was
> >> a bit puzzled by the outcome as I do not understand the scale of the 
parameter
> >> estimates. I specified the outcome model and mediator models as
> >> follows:
> >>
> >> # AFT model for time to death
> >> model.y<- survreg(Surv(weekstodeath_ssn_dob,mortality_status_dob)
> >>              ~z_a + pat_sex + tribaseage + grade1 + diabetic +
> >>                cvd_broa + selfhealth_1 + adlbase + iadlbase + smoking2
> >>                + z_n + z_e + z_o + z_c,dist="weibull",data=ffi_mediator)
> >>
> >> # Create polr model for predicting smoking from Agreeableness
> >> model.m<- (polr(data=ffi_mediator,Hess=T,method="probit",smoking2 ~ z_a 
+
> >> pat_sex
> >>                  + tribaseage + grade1 + diabetic + cvd_broa +
> >>                  selfhealth_1 + adlbase + iadlbase + z_n + z_e + z_o + 
z_c))
> >>
> >> and used the following for the mediate() function:
> >>
> >> smoking_agreeableness_covariates<-
> >>   mediate(model.m,model.y,sims=1000,treat="z_a",mediator="smoking2",
> >>   covariates=list("pat_sex","tribaseage","grade1","diabetic1","cvd_broa",
> >>   "selfhealth_1","adlbase","iadlbase","z_n","z_e","z_o","z_c"))
> >>
> >> The results of the analyses are as attached as a text file.
> >>
> >> My question concerns the parameter estimates obtained. First, I cannot 
quite
> >> work out what metric they are on as they are not in the same as the AFT
> >> analysis. Moreover, I am not wholly clear on how to interpret the results
> >> given that they seem to be for cases in which the treatment variable is
> >> categorical.
> >>
> >> I appreciate you are busy, but I would most appreciate your assistance in 
this
> >> matter.
> >>
> >> Sincerely,
> >>
> >> Alex
> >>
> >>
> >>
> >> --
> >> The University of Edinburgh is a charitable body, registered in
> >> Scotland, with registration number SC005336.
> >>
> >> <mediation_results_11_april_2012.txt>
> >
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information
> 
> -- 
> ====================================
> Teppei Yamamoto
> Assistant Professor
> Department of Political Science
> Massachusetts Institute of Technology
> http://web.mit.edu/teppei/www/
> ====================================
> 
> 

-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.