<div dir="ltr"><div>Dear Tabea,</div><div><br></div><div> 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.</div><div><br></div><div>Best,</div><div>Kosuke</div><br><div class="gmail_extra"><div><div class="gmail_signature"><div dir="ltr">---------------------------------------------------------<br>Kosuke Imai Office: Corwin Hall 036<br>Professor Phone: 609-258-6601<br>Department of Politics Fax: 609-258-1110<br>Princeton University Email: kimai@Princeton.Edu<br>Princeton, NJ 08544-1012 <a href="http://imai.princeton.edu">http://imai.princeton.edu</a><br>---------------------------------------------------------<br><br></div></div></div>
<br><div class="gmail_quote">On Thu, Dec 11, 2014 at 10:00 AM, Schoeler, Tabea <span dir="ltr"><<a href="mailto:tabea.t.schoeler@kcl.ac.uk" target="_blank">tabea.t.schoeler@kcl.ac.uk</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal">Dear Prof Kosuke Imai,<u></u><u></u></p>
<p class="MsoNormal">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.
<u></u><u></u></p>
<p class="MsoNormal">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).<u></u><u></u></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Causal
Mediation Analysis <u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Nonparametric
Bootstrap Confidence Intervals with the Percentile Method<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial"><u></u> <u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">
Estimate 95% CI Lower 95% CI Upper p-value<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ACME
(control) 0.29410 0.13608 0.49965 0.00<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ACME
(treated) 0.18529 0.10937 0.25479 0.00<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ADE
(control) -0.07367 -0.13916 0.00882 0.07<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ADE
(treated) -0.18247 -0.42868 0.01485 0.07<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Total
Effect 0.11163 0.02287 0.20054 0.02<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Prop.
Mediated (control) 2.63464 0.84232 13.87867 0.02<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Prop.
Mediated (treated) 1.65993 0.91547 5.70383 0.02<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ACME
(average) 0.23970 0.12452 0.36121 0.00<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">ADE
(average) -0.12807 -0.28627 0.01183 0.07<u></u><u></u></span></p>
<p class="MsoNormal" style="margin-bottom:0.0001pt;word-break:break-all"><span style="font-size:10pt;font-family:'Lucida Console';background-color:rgb(225,226,229);background-repeat:initial initial">Prop.
Mediated (average) 2.14728 0.87885 9.53672 0.02<u></u><u></u></span></p>
<p class="MsoNormal"><u></u> <u></u></p>
<p class="MsoNormal">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.<u></u><u></u></p>
<p class="MsoNormal">King regards,<u></u><u></u></p>
<p class="MsoNormal">Tabea Schoeler<u></u><u></u></p>
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<div><span style="color:rgb(54,95,145);font-size:10pt">Tabea Schoeler (PhD student)</span></div>
<div><span style="font-size:10pt;color:rgb(54,95,145)">Department of Psychosis Studies, Institute of Psychiatry, </span><span style="color:rgb(54,95,145);font-size:13px">Psychology & Neuroscience, </span></div>
<div><font color="#365f91">King’s College London</font></div>
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