By-election vote tracker
A running estimate of party support built from council by-elections.
How it's calculated
For each by-election: each party's share change since the ward was last fought in May. The tracker for a date = the latest National Share Estimate + the recency-weighted mean change across by-elections in the previous 3 months (weight 1 today, falling to 0.25 at 3 months; equal weight per contest). At least 15 contests are needed; otherwise we show "Not enough by-elections yet".
Worked example
One input from Heworth, 15 January 2026: Labour -27.5pp, Green +4.4pp, Reform UK +20.1pp (did not stand in 2023, counted from 0 and flagged).
Source: Heworth ward, City of York — by-election of 15 January 2026 and May 2023.
Caveats
- By-elections have low turnout and local factors; the tracker smooths across many contests.
Where it's used
- By-election Vote Tracker (coming soon)
Other definitions
Vote shareChange in shareSwing (two-party, Butler)MajoritySwing 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 EstimateNotional resultIndex of changeSplit-ticket indicatorNational Equivalent Vote