[Data Mining in Politics]

Similarity Networks

Clustering does not illustrate the structure of strong similarities in detail. We can achieve this by plotting a graph with nodes corresponding to senators and edges to their connections. We only select a certain number of the strongest similarities to create a graph, using an artificial threshold to discriminate between a connection and the absence of it. We graphically illustrate the 20 pairs of senators with highest Rajski's distance between their votes, both in the Democratic and Republican Party. The nodes are labeled with the total number of votes cast, while the edges are marked with the percentage of roll calls in which both senators cast the same vote

Among the Democrats, there is large variation in the number of votes cast, as those senators that later participated as candidates in the presidential race cast fewer votes than others, with Senator Kerry casting only 165 votes out of 459. This seemingly places him in the very center of the party. The closest pair among the Republican senators is formed by Senators Craig and Crapo, both from Idaho, who cast identical votes in 98.8% of cases. We also see the separation between the two Republican blocs.