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

[Kosuke Imai]

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>