Moderation with a multicategorical moderating variable

This post was was inspired by Nicholas Michalak’s Novum R-ganum blog posts on reproducing Hayes’ PROCESS Model 1 in R. He has two posts where he presents the R code for examining a continuous × continuous moderation and a dichotomous × continuous moderation.

However, a quick Google search suggests a paucity of information for conducting moderation analyses using a multicategorical moderating variables.

Therefore, this post will outline how to run the PROCESS Model 1 with a multicategorical moderator (M) in R. We will examine how to code the M variable,  simulate some data, run the PROCESS analyses in both SPSS and R, and compare the results from both software packages.

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Promoting an open research culture

You may have guessed from my last post that I am an advocate for transparency, openness, and reproducibility in research. Part of this involves making data and code publically available.

Nosek and colleagues (2015) published an article in Science outlining the Transparency and Openness Promotion (TOP) guidelines and is well worth a read.

It is available from Science or, if you don’t have access to Science, it’s available from ResearchGate.

Nosek, B.A., Alter, G., Banks, G.C., Borsboom, D., Bowman, S.D., Breckler, S.J., . . . Yarkoni, T. (2015). Promoting an open research culture. Science, 348, 1422-1425. doi: 10.1126/science.aab2374