Directional separation (d-separation) is a graphical criterion used to determine whether a set of variables is conditionally independent of another set in a directed acyclic graph (DAG). In a directed graph, two variables $X$ and $Y$ are d-separated by a set of conditioning variables $Z$ if there is no active path connecting them, meaning they are conditionally independent given $Z$. Understanding d-separation is essential for proper experimental design and observational analysis to prevent biases, such as post-treatment bias, which occurs when conditioning on an intermediate variable (like a mediator) blocks the true causal pathway or introduces spurious associations.