The Sobel test is used to test what in mediation analysis?

• A. A test of the overall model fit.
• B. A significance test of mediation, testing the indirect effect.
• C. A test for equality of variances.
• D. A test for normality of residuals.

Answer: (B)

The Sobel test is about testing whether mediation is actually occurring by examining the significance of the indirect effect. In mediation, you’re looking at how much of X’s impact on Y is transmitted through a mediator M. The indirect effect is the product ab (a is the effect of X on M, b is the effect of M on Y while controlling for X). The Sobel test uses the standard errors of these paths to compute a z-statistic for ab and tests whether that indirect effect is different from zero. If it’s significant, that provides evidence that the mediator carries part of the effect from X to Y. This is not a test of overall model fit, nor of equal variances or residual normality, which are addressed by other diagnostics.