Psephology.AI

    Change in share

    How far a party's vote share moved since the same ward was last fought.

    How it's calculated

    Share now minus share last time, in percentage points (pp). Only where the ward's boundaries have not changed, or against a notional result.

    Worked example

    Labour in Heworth: 36.7% at the 2026 by-election, 64.3% in May 2023.

    Change = -27.5pp. Reform UK did not stand in 2023, so it counts from 0: +20.1pp.

    Source: Heworth ward, City of York — by-election of 15 January 2026 and May 2023.

    Caveats

    • A party that did not stand last time counts from 0 and is flagged.
    • Comparing a by-election with a multi-seat May contest mixes one-vote and several-vote ballots.

    Where it's used

    Other definitions

    Vote 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 EstimateBy-election vote trackerNotional resultIndex of changeSplit-ticket indicatorNational Equivalent Vote