I used the steps in the sidebar beneath this to find how frequently the Democratic and Republican parties voted the same way on various issues in 1990, 2000, 2010.
Note: This analysis uses the same data-source as Part 1; explanations from this part will pick up where Partisanship Ratios for Each Vote left off.
The Agreement Ratio
This is a simple concept-- how often did the two parties agree? I calculated a ratio of agreement by dividing "Issues that Both Parties Agreed On" by "Total Issues Voted On." A ratio of 1 would mean that the majority of each party always voted the same way as the majority of the other, while 0 would mean that neither majority ever voted the same way as the other.
There were 50 votes this year; Democratic and Republican majorities voted the same way for 30 of them, but in different ways for 20 of them.
The agreement ratio therefore is 30/50, or .6. In other words, Democratic and Republican majorities agreed 60% of the time.
Why Look at Agreement, Not Just the Partisanship Ratio?
The Partisanship Ratio indicates how often Senators choose to vote against their party, an important aspect of partisanship.
Partisanship as it's used in political media, however, has a somewhat larger scope than just this. When we say that "partisanship is increasing," we don't just mean that Senators from each party are refusing to make compromises (raising Partisanship Ratios through party loyalty), but that the parties themselves are also disagreeing on more things. Assuming that a relatively equivalent set of bills comes through Congress each session, falling agreement ratios might indicate that entire political parties are starting to take stances against each other because of partisanship rather than because of a bill's impact.
To sum up:
To continue, go to Calculating an Agreement Ratio.