Based on the causal diagram below, where:
- Y1 is the outcome in the post-intervention period,
- Y0 is the pre-intervention outcome,
- X is a Yes/No healthcare intervention, and
- Z1 & Z2 represent various confounder variables (diagnoses, geography, etc.).
Shouldn't Y0 be excluded from the counterfactual model estimating the effect of X on Y1 given it is both a collider and mediator variable?
Is there any reason not to exclude Y0 from the study?