What is the Single Transferable Vote?

The Single Transferable Vote (STV) is a preferential system of proportional representation that has long been a favourite of many British electoral reformists, including being the voting system of choice of the Liberal Democrats, the SNP and Plaid Cymru.

Developed independently in the 1850s by Danish mathematician Carl Andræ and British lawyer Thomas Hare, STV today is used for pretty much all elections in Ireland and Malta, as well as elections to the Australian Senate and some of the country’s state legislative chambers. In the UK, it is used for devolved and local elections in Northern Ireland, local elections in Scotland, with councils in Wales currently considering switching to STV from 2027.

How does it work?

STV elections take place in multi-member constituencies, typically electing between around three and six members. 

Voters rank candidates (1, 2, 3, etc.) in order of preference, usually being able to rank as many or as few as they like.

To allocate seats, a quota is set – typically this is the ‘total number of votes’ divided by the ‘total number of seats, plus one’, plus one. So, if 5,000 votes had been cast in a four-member constituency, the quota would be 1,001. 

If a candidate wins more votes than this quota, they are elected. Any surplus votes (to continue the example, if a candidate won 1,021 votes, there would be 20 surplus) are then transferred to the next preference indicated on the ballot paper. There are two main ways of deciding which votes are the ‘surplus’ – the random method, where 20 votes would be randomly chosen (used in Ireland), or the fractional method, where all 1,021 votes would be weighted to represent 20 votes (used in Northern Ireland and Scotland). 

If no candidates win more votes than the quota, the candidate with the fewest votes is eliminated and their votes are transferred to their voter’s next preference. This continues until all seats have been filled.

Although votes are cast directly for candidates, rather than primarily for a party as in other forms of PR, STV nevertheless delivers a broadly proportional relationship between a party’s overall vote share and seat share. But, as voter’s wider preferences have an impact on the result, parties that perform better on non-first preferences (i.e., are broadly popular) will get a small boost in representation, when compared to an equivalent List PR system.

Why STV? / What Supporters Say

STV maximises voter choice, giving voters free rein to express preferences between as many candidates (and implicitly parties) as they like. In other words, voters can make it as clear as possible who they want elected and who they don’t, with STV’s focus on candidates encouraging voters to engage more with these questions.

In combination with relatively small constituencies, STV’s focus on candidates additionally ensures that a strong constituency link is maintained. Indeed, a 1997 study found that Irish MPs actually dedicate more time to constituency work than their British counterparts. This ability to mix proportionality and strong local links, so say STV advocates, makes it particularly well suited to local government.

Additionally, due to its preferential nature, STV is said to encourage moderation and positive campaigning. This is because it is necessary for candidates and parties not just to win as many first preference voters as you can, but to win the highest possible preference votes of other parties’ supporters. 

Variations

Like any proportional system, certain elements of the system can be modified, including exactly which quota you use or how surplus votes are determined. Constituency size can be varied too – the Irish parliament uses constituencies of between three and five members, council electoral districts in Northern Ireland vary between electing five and seven members, while Malta and the Northern Irish Assembly use exclusively five-member constituencies. Larger constituencies increase the proportionality of STV, but can reduce localness and lead to longer ballot papers. 

Apart from these small variations, most STV systems are broadly similar. The only really particularly unusual variants have come in Australia, where their larger constituency sizes – Western Australia uses a single statewide 37-seat district – and rules on ranking all candidates left many voters frustrated at having to rank potentially over a hundred candidates! As such, they created the ‘Above the Line’ variant, which allows voters to simply rank parties instead of candidates – effectively STV with party lists.

With thanks to Dylan Difford for compiling this explainer. Dylan is a researcher specialising in elections and voting systems, as well as formerly a regular contributor to the Electoral Reform Society blog.