What it measures
The center aisle of each chamber is a tradition as old as the seating chart. X counts, on roll calls where the two caucus majorities took opposite sides, how often a member voted with the other side of the aisle.
By the numbers
| Measure | Value | Source |
|---|---|---|
| House roll calls on which the caucus majorities did NOT take opposite sides, 1970 | 72.9% (100 − 27.1) | Brookings, Vital Statistics on Congress, Table 8-3 (from CQ party-unity studies), computed |
| Same, House, 2017 | 24.0% (100 − 76.0) | Brookings, Vital Statistics on Congress, Table 8-3 (from CQ party-unity studies), computed |
| Member-level cross-aisle rates by Congress | data needed | not yet sourced |
Fewer split roll calls means fewer chances to cross, so X naturally runs small. Full formula: Methodology.