Jeromy Anglim's Blog: Psychology and Statistics


Friday, November 7, 2008

Getting Started with Mediation and Moderation: SPSS and R Resources

The following post sets out resources for getting started with mediation and moderation analysis.

Some Resources

SPSS Resources

R Mediation Resources

R Moderation Resources

  • To examine moderation is to examine an interaction effect. R makes it very easy to examine interaction effects with its standard model notation (see William Revelle's site for a discussion of lm and model notation).
  • moderate.lm in QuantPsyc package

Why I often shudder when I get asked about mediation and moderation in statistical consulting sessions

Mediation

Researchers testing for mediation (IV to MV to DV) often make causal conclusions when none are warranted. At the very least researchers should consider alternative causal pathways.

Moderation

Testing for moderation can be sexy in some disciplines in a way that testing simple main effects is not. However, in most observational studies that I encounter in the social sciences, interaction effects tend to be small if present at all. Researchers should not lose site of the importance of main effects in the quest to find supposedly sexy interactions.

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