It's important for users to understand what our scatter plots are showing. The horizontal and vertical positions of the circles on the plots are completely independent.
We developed an original approach for statistical analysis of bill sponsorship and cosponsorship. Most bills have a single sponsor attached to them. This sponsor is generally the person who authored the bill. Bills can have many cosponsors who join later.
For each bill, we compute a representative’s leadership on the following basis:
- Sponsors receive 1 point.
- Each cosponsor receives a fraction of 1 point,
depending on how far along the bill was in the process when they joined as a cosponsor.
For cosponsors, we look at the entire duration that the bill had any updates or actions taken on it and we assign points accordingly. Actions include voting on a bill, referring a bill to a committee, and joining a bill as a cosponsor. For example, if the period between a bill’s introduction and the last action taken on it is 30 days and a representative joins as a cosponsor after 10 days, the representative receives 0.333 points.
To compute a person’s position for a selected period we take the average of the representative’s scores for all bills the person sponsored or cosponsored during the given period. Therefore, sponsoring or cosponsoring numerous bills yields a high score, placing the representative near the top of the spectrum. Those representatives who sponsor and cosponsor bills infrequently, or take a long time to cosponsor bills appear lower on the spectrum. The result is a value between 0 and 1 for each unique pair of dates on which voting took place for each representative. We scale these values to the height of the spectrum.
One bias we acknowledge is that the presentation of this analysis as “Leaders” and “Followers” imposes a value judgment on the data. We label the axis with “Leaders” and “Followers” for simplicity.
We calculated Poole-Rosenthal scores using a program provided by Keith T. Poole called W-NOMINATE (http://voteview.ucsd.edu/w-nominate.htm). For each unique pair of selectable dates we run WNOMINATE on all roll call votes cast during that period. We used Poole-Rosenthal scores for the horizontal positions of representatives.