Solution
= Solution
The $X_1$ coefficient is the total causal effect of $X_1$ on $Y$. The node $X_1$ has no parents, so there is no <backdoor path> into it, and the unadjusted regression sums all directed paths from $X_1$ to $Y$.
= Solution
The $X_1$ coefficient is the total causal effect of $X_1$ on $Y$. The node $X_1$ has no parents, so there is no <backdoor path> into it, and the unadjusted regression sums all directed paths from $X_1$ to $Y$.