Revisiting equivalent structural models through the mediation models’ Markov equivalence class

Rational inquiry in psychology aims not just to describe phenomena but also to explain them. Researchers continue to use the classic trivariate mediation model as a key tool in this pursuit. The problem of equivalent structural models is revisited as a Markov equivalence class (MEC) ( Andersson et al., 1997 ), illustrated using the trivariate mediation model. We highlight how the MEC complicates causal interpretation of the classic mediation model. To illustrate this, we examine two recent articles from a widely read psychology journal that apply the classic trivariate mediation model, one demonstrating strong causal control, the other weak causal control. We also introduce an R Shiny application that helps users understand this linkage and explore how causal assumptions shape the number of equivalent models in the MEC of a mediation model.