on illustrations and simulations in which the correlation between trait T and moderator M runs either exclusively via A, or via C or via E. Although these settings may be considered quite special, they conveniently simplify the explanation of the problem of non-zero semi-partial correlations in the univariate moderation model proposed by Purcell, and clarify how this model would need to be extended. In the subsequent investigation of the usefulness of this extended version of the univariate model, the simulations are extended to more realistic conditions.
