# Relationship between anabolic and catabolic pathways

Type 1 and type 2 represent the same statistical dependencies (., X {\displaystyle X} and Z {\displaystyle Z} are independent given Y {\displaystyle Y} ) and are, therefore, indistinguishable within purely cross-sectional data . Type 3, however, can be uniquely identified, since X {\displaystyle X} and Z {\displaystyle Z} are marginally independent and all other pairs are dependent. Thus, while the skeletons (the graphs stripped of arrows) of these three triplets are identical, the directionality of the arrows is partially identifiable. The same distinction applies when X {\displaystyle X} and Z {\displaystyle Z} have common ancestors, except that one must first condition on those ancestors. Algorithms have been developed to systematically determine the skeleton of the underlying graph and, then, orient all arrows whose directionality is dictated by the conditional independencies observed. [27] [30] [31] [32]