Figure 4.
Example of pruning. In panel (a), the graph is a chain of alternate treated units and control units. There are three removable edges (t2, c1), (t2, c2), (t3, c2). We pick the removable edge with the maximal distance, i.e., (t2, c2), eliminate the edge and obtain panel (b). After pruning (t2, c2), edges (t2, c1) and (t3, c2) are no longer removable, and the set of removable edges is empty. Therefore, we stop pruning. In the pruned match, units can be split into two connected subgroups: {t1, t2, c1} and {t3, c2, c3}. The connected subgroups either consist of one treated unit and multiple control units or vice versa.