Visualization of examples of the 6 types of Markov Equivalence Classes (MECs), their corresponding essential graphs, the DAGs contained in those essential graphs, the total number of DAGs that fall into each type of essential graph, and the quantity of undirected edges with equal and unequal odds of being oriented in either direction, given a uniform distribution over the DAGs.

In addition to undirected hub graphs, undirected chain graphs also contain undirected edges which are more likely to be oriented in one direction. With 4 nodes and 3 edges, other than 50% edges there are only 75% edges and completely directed edges. Overall, 46% of the undirected edges are 75% edges, indicating that non-50% edges are common among graphs with 4 nodes and 3 edges.