Swing (two-party, Butler)
The movement between two named parties, averaged over both.
How it's calculated
((A now − A before) − (B now − B before)) ÷ 2. We always name both parties; a bare swing number is never shown.
Worked example
Green: 15.4% → 19.8%. Labour: 64.3% → 36.7%.
Swing from Labour to Green = 16.0pp.
Source: Heworth ward, City of York — by-election of 15 January 2026 and May 2023.
Caveats
- Swing describes two parties only; with many parties in play it hides a lot.
Where it's used
Other definitions
Vote shareChange in shareMajoritySwing needed to gainUncontested seatCandidates per seat / per wardContestation rateWinner's shareEffective number of partiesWinner's bonus (seats vs votes)Incumbency and re-election rateWomen candidates and councillorsTurnout (three methods)Council control and No Overall Control (NOC)Majority at riskPsephology National Share EstimateBy-election vote trackerNotional resultIndex of changeSplit-ticket indicatorNational Equivalent Vote