Path-specific effects (PSEs) are a critical measure for assessing mediation in the presence of multiple mediators. However, the conventional definition of PSEs has generated controversy because it often causes misinterpretation of the results of multiple mediator analysis. For in-depth analysis of this issue, we propose the concept of decomposing fully mediated interaction (FMI) from the average causal effect. We show that FMI misclassification is the main cause of PSE misinterpretation. Two strategies for specifying FMI are proposed: isolating FMI and reclassifying FMI. The choice of strategy depends on the objective. Isolating FMI is the superior strategy when the main objective is elucidating the mediation mechanism whereas reclassifying FMI is superior when the main objective is precisely interpreting the mediation analysis results. To compare performance, this study used the two proposed strategies and the conventional decomposition strategy to analyze the mediating roles of dyspnea and anxiety in the effect of impaired lung function on poor health status in a population of patients with chronic obstructive pulmonary disease. The estimation result showed that the conventional decomposition strategy underestimates the importance of dyspnea as a mechanism of this disease. Specifically, the strategy of reclassifying FMI revealed that 50% of the average causal effect is attributable to mediating effects, particularly the mediating effect of dyspnea.
Tai, An-Shun; Liao, Le-Hsuan; and Lin, Sheng-Hsuan, "On The Conventional Definition Of Path-Specific Effects - fully mediated interaction with multiple ordered mediators" (July 2021). Harvard University Biostatistics Working Paper Series. Working Paper 229.