[Mediation-information] mediation in R

[Kosuke Imai]

Dear Tabea,

  Your description seems to indicate that there is a strong relationship
(perhaps by construction) between T and M.  So, I wouldn't be surprised
that even if the relationship between M and Y conditional on T is rather
weak the ACME may come out to be statistically significant.  Now, since
your mediator is ordinal variable, you may want to treat it as such rather
than using it in the linear model (assuming that's what you did).   Good
luck with your research.

Best,
Kosuke

---------------------------------------------------------
Kosuke Imai               Office: Corwin Hall 036
Professor                 Phone: 609-258-6601
Department of Politics    Fax: 609-258-1110
Princeton University      Email: kimai at Princeton.Edu
Princeton, NJ 08544-1012  http://imai.princeton.edu
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On Thu, Dec 11, 2014 at 10:00 AM, Schoeler, Tabea <
tabea.t.schoeler at kcl.ac.uk> wrote:

>  Dear Prof Kosuke Imai,
>
> I am a PhD student at King’s College London using the mediation package
> for R. I am approaching you because I have a little question regarding the
> assumptions that need to be fulfilled to run the proposed mediation
> analysis. I can fully understand if you are too busy to respond to this
> mail, but in case you had 5 spare minutes, I would be immensely grateful
> for any advice in this regard.
>
> I have the dichotomized independent variable IV = cannabis use (yes if
> used at least once out of 3 time points / no if never used) as the
> predictor variable for risk of violence. I created the continuous mediator
> variable M = frequency of use, ranging from 0 (used at 0 time points) up to
> 3 (used at 3 time points). When I included M in the logistic regression
> model, the effect of IV cannabis use (Yes/No) was no longer a significant
> predictor for violence. The mediation effect was significant when tested in
> R (cf. Table below).
>
> Causal Mediation Analysis
>
>
>
> Nonparametric Bootstrap Confidence Intervals with the Percentile Method
>
>
>
>                          Estimate 95% CI Lower 95% CI Upper p-value
>
> ACME (control)            0.29410      0.13608      0.49965    0.00
>
> ACME (treated)            0.18529      0.10937      0.25479    0.00
>
> ADE (control)            -0.07367     -0.13916      0.00882    0.07
>
> ADE (treated)            -0.18247     -0.42868      0.01485    0.07
>
> Total Effect              0.11163      0.02287      0.20054    0.02
>
> Prop. Mediated (control)  2.63464      0.84232     13.87867    0.02
>
> Prop. Mediated (treated)  1.65993      0.91547      5.70383    0.02
>
> ACME (average)            0.23970      0.12452      0.36121    0.00
>
> ADE (average)            -0.12807     -0.28627      0.01183    0.07
>
> Prop. Mediated (average)  2.14728      0.87885      9.53672    0.02
>
>
>
> My question is now whether I am allowed to use the frequency variable as a
> mediator since only subjects in the cannabis YES group would be exposed to
> the ranges 1-3, while all subjects in the cannabis NO group would have the
> score 0 in the mediator variable. This is to test whether the effect of
> cannabis use on violence is driven by its frequency of use.
>
> King regards,
>
> Tabea Schoeler
>  Tabea Schoeler (PhD student)
> Department of Psychosis Studies, Institute of Psychiatry, Psychology &
> Neuroscience,
> King’s College London
>
>
>
>
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