Table 1.

Comparison Between the SEM and PS Approaches

Aspect	SEM approach	PS approach
Key modeling difference	Focus on observed mediator status	Focus on potential mediator status
Key strength	Compatibility with the mediational process theory	Clarity in underlying assumptions necessary for causal interpretation
Frequent context	Very general, theory-based mediation	Treatment noncompliance
Fixed assumptions for causal interpretation	Ignorability of mediator status	None
Frequently used assumptions for causal interpretation	Ignorability, constant effect, linearity	Monotonicity, exclusion restriction
Key method of checking validity of causal effect estimates	Test the fit of the model to the data	Test sensitivity of estimates to violating underlying assumptions

Note. SEM = structural equation modeling; PS = principal stratification.