Effective number of parties
How many parties are really competing, adjusting for their size.
How it's calculated
Laakso-Taagepera: 1 ÷ the sum of each party's share squared (shares as fractions). Two equal parties = 2.0; one dominant party ≈ 1.
Worked example
Heworth May 2023: 2.2.
Heworth 2026 by-election: 4.0, a much more fragmented contest.
Source: Heworth ward, City of York — by-election of 15 January 2026 and May 2023.
Caveats
- Uses the vote-share rule for multi-member wards.
Where it's used
- Election Health (coming soon)
Other definitions
Vote shareChange in shareSwing (two-party, Butler)MajoritySwing needed to gainUncontested seatCandidates per seat / per wardContestation rateWinner's shareWinner'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