Solution
= Solution
The $X_2$ coefficient has no causal interpretation. The open backdoor path $X_2\leftarrow X_1\to Y$, together with paths through $X_3$, confounds the unadjusted association.
= Solution
The $X_2$ coefficient has no causal interpretation. The open backdoor path $X_2\leftarrow X_1\to Y$, together with paths through $X_3$, confounds the unadjusted association.