Quantifying Inconsistent Mediation: The Mediated Magnitude Share and the Degree of Suppression
Year: 2026
Type: preprint
Abstract: The proportion mediated (PM), the indirect effect divided by the total effect, is intuitive when direct and indirect effects act in the same direction. Under inconsistent mediation, however, opposing pathways partially cancel, so PM can fall outside the unit interval and becomes unstable as the total effect approaches zero. We propose two complementary effect-scale measures: the mediated magnitude share (MMS), which quantifies the absolute magnitude of an indirect pathway relative to the combined absolute magnitudes of the direct and indirect pathways, and the degree of suppression, which quantifies the fraction of this combined magnitude cancelled by opposing pathways. Both measures are bounded between 0 and 1, and MMS reduces to PM under consistent mediation. We extend both measures to multiple indirect pathways and, through a logarithmic transformation, to binary outcomes on ratio scales. In simulations with continuous and binary outcomes, PM estimates were highly dispersed when the total effect was small relative to the pathway magnitudes, whereas estimates of MMS and the degree of suppression remained bounded. We illustrate the measures using previously published estimates from the Adolescents Training and Learning to Avoid Steroids (ATLAS) program, where PM was $-0.30$, MMS was $0.19$, and the degree of suppression was $0.38$, showing how the two measures describe the relative pathway magnitudes and their cancellation under inconsistent mediation.
Source: arXiv (Cornell University)
Language: English
Subfield: Statistics and Probability
Field: Mathematics
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7214775363