How the Single Transferable Vote (STV) system works

A members of the public arrives at Brookeborough Primary School in County Fermanagh which is one of the polling stations in the Northern Ireland Assembly elections.

Photo by Kelvin Boyes / Press Eye

A members of the public arrives at Brookeborough Primary School in County Fermanagh which is one of the polling stations in the Northern Ireland Assembly elections. Photo by Kelvin Boyes / Press Eye

While the make-up of the next Assembly chamber remains in doubt, there will definitely be fewer bums on seats.

The snap election has brought forward plans to reduce the number of members from 108 to 90.

Under a law change that would have been implemented in 2021 - had the last administration lasted five years rather than eight months - the poll will return five MLAs in each of the region’s 18 constituencies - down from six members previously.

Elections to the Assembly use the Single Transferable Vote (STV) system, a form of proportional representation involving voting for candidates in order of preference.

This system aims to reduce the so-called “wasted votes” seen in the UK Parliament’s first-past-the-post system, where many electors in traditionally “safe” constituencies find their chosen party never wins and their vote does not count towards electing an MP, or it always wins, piling up majorities of thousands of votes above the number actually needed for victory.

Under STV, instead of a majority of votes, the winning post for the candidates is the quota - the number of valid votes cast divided by the number of seats plus one, plus one vote.

In all of the Northern Ireland constituencies, this comes to a sixth (previously a seventh) of the votes plus one vote. Any candidate passing the quota at any stage during the counting is elected.

Voters are asked to number candidates on the ballot paper in order of preference. The count then involves distributions of votes through many rounds.

Candidates passing the quota are declared elected and the number of their votes above the quota are then distributed to others in accordance with their supporters’ lower preferences as they have expressed on the ballot paper.

Low-scoring candidates are progressively eliminated, with their votes distributed to others in accordance with their supporters’ lower preferences.

This means that the five people elected in each of the 90 seats will be either candidates passing the quota or those remaining when all the other candidates have been eliminated.

Example of a count of first preference votes in a five-member constituency with fictitious candidates:

Peter Johnson (SF) 7,001

Ann Carr (DUP) 6,201

Bill Moffatt (SF) 6,110

John Harper (SF) 5,476

Richard Miller (UUP) 4,910

Paul Jones (SDLP) 4,002

Julie O’Malley (SDLP) 3,445

Ann Kelly (UUP) 1,004

Ann Brookes (Ind) 877

David Hopkins (ND) 845

Martin Thompson (Ind) 598

John Arnott (Alliance) 556

William Jones (UKIP) 411

Danny Gibson (Green) 287

The turnout - the number of valid votes cast - is 41,723 and the quota is set at one-sixth of that, 6,594 plus one vote to be elected.

One candidate, Peter Johnson, is above the quota and therefore is declared elected at this stage.

The count then moves on to a second round where Mr Johnson’s surplus of 407 votes is redistributed to other candidates in accordance with his supporters’ second choices as they have expressed on the ballot paper.

The result of the second count in this fictional example is:

Ann Carr (DUP) 6,201

Bill Moffatt (SF) 6,337

John Harper (SF) 5,586

Richard Miller (UUP) 4,910

Paul Jones (SDLP) 4,003

Julie O’Malley (SDLP) 3,447

Ann Kelly (UUP) 1,004

Ann Brookes (Ind) 877

David Hopkins (ND) 847

Martin Thompson (Ind) 598

John Arnott (Alliance) 557

William Jones (UKIP) 411

Danny Gibson (Green) 289

Nobody new has passed the quota this time. The lowest-scoring candidate, Danny Gibson, is now eliminated from the contest and his votes may be redistributed in a subsequent round to other candidates on the basis of voters’ lower preferences.

If there are a number of candidates at the bottom of the tally whose total vote is insufficient to make further progress, they could all be eliminated at the same time.

This process continues into further rounds of counting, with candidates either passing the quota and being elected or being eliminated from the bottom of the tally.

An example of a final - and in this case 10th count - where three candidates have passed the quota in earlier rounds and have been declared elected, with just two places remaining:

Distribution of Julie O’Malley’s votes after she was eliminated from the contest in the previous (9th) round:

John Harper (SF) 5,998

Richard Miller (UUP) 6,003

Paul Jones (SDLP) 6,025

None of the remaining candidates has cleared the quota but the two highest-scoring ones, Paul Jones and Richard Miller, are declared elected while the lowest-scoring candidate, John Harper, is eliminated from the contest.

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