Election buffs could be in for a long night on Friday as they wait to find out the make-up of the next Northern Ireland Assembly.

The poll will see 90 MLAs elected to Stormont – five in each of Northern Ireland’s 18 constituencies. The counting will take place in three centres in the region.

Elections to the Assembly use the single transferable vote (STV) system, a form of proportional representation involving choosing 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 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 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 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 lower preferences.

This means 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) 7,001Ann Carr (DUP) 6,201Bill Moffatt (SF) 6,110John Harper (SF) 5,476Richard Miller (UUP) 4,910Paul Jones (SDLP) 4,002Julie O’Malley (SDLP) 3,445Ann Kelly (UUP) 1,004Ann Brookes (Ind) 877David Hopkins (ND) 845Martin Thompson (Ind) 598John Arnott (Alliance) 556William Jones (Ukip) 411Danny Gibson (Green) 287

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

One candidate, Peter Johnson, is above the quota and 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 expressed on the ballot paper.

– The result of the second count is:

Ann Carr (DUP) 6,201Bill Moffatt (SF) 6,337John Harper (SF) 5,586Richard Miller (UUP) 4,910Paul Jones (SDLP) 4,003Julie O’Malley (SDLP) 3,447Ann Kelly (UUP) 1,004Ann Brookes (Ind) 877David Hopkins (ND) 847Martin Thompson (Ind) 598John Arnott (Alliance) 557William Jones (Ukip) 411Danny Gibson (Green) 289

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

If there are a number of candidates at the bottom of the tally whose total votes are 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 (ninth) round:

John Harper (SF) 5,998Richard Miller (UUP) 6,003Paul 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, John Harper, is eliminated.