What is the Supplementary Vote (SV)?
The Supplementary Vote, or SV, is a majoritarian, preferential voting system. This means that voters rank their two favourite candidates in order of preference, which is then used to elect a single winner via two rounds of vote counting.
The aim of this electoral system is that winning candidates must attract support from a wide range of voters. Whereas in FPTP, the winning candidate does not need to achieve a majority of votes, candidates in an SV election must get more than 50% of first preference votes to win outright.
If no candidate achieves this, all but the top two candidates are eliminated, and the second preference votes are reallocated. The candidate with the highest total of first and second preference votes wins.
What elections use Supplementary Vote?
SV, where voters rank two candidates only, is one version of a broad group of ‘preferential’ voting systems, which include the Alternative Vote (AV) used in Australia (voters rank as many candidates as they like), and the Contingent Vote used to elect the President of Sri Lanka (voters rank their three favourite candidates).
How do I vote under Supplementary Vote?
For elections using SV, voters are given a ballot paper with two columns next to the names of all candidates standing for election.
In the first column, voters mark their favourite candidate with a cross (‘X’) inside the box next to their name. In the second column, voters put a cross to mark their second favourite candidate.
Voters do not have to choose a second favourite candidate if they do not have one. There is no benefit in choosing the same candidate for both options.
Counting the votes
First preference votes are counted first. If a candidate receives over 50% of first preference votes, they are immediately elected.
However, if no candidate has received a majority of first preference votes, everyone except the top two candidates is eliminated from the race.
Ballot workers then count the second preference votes of all the eliminated candidates. If any of these votes are for the top two candidates, they are added to those candidates’ total votes.
Any second preference votes for already eliminated candidates are disregarded. The remaining candidate with the most first and second preference votes is declared the winner.
Simplicity: The Supplementary Vote only requires one round of voting in an election, as opposed to other preferential systems that require a second round of voting for remaining candidates. Also, it only asks voters to rank their two favourite candidates, rather than potentially ranking all of the candidates in an election, as is possible with the Alternative Vote for example.
Positive campaigning: Candidates are encouraged to seek support beyond their core base of supporters by trying to win the second preference votes of other candidates, therefore creating a less confrontational approach to political campaigning.
Voting for ‘third-party’ candidates: Since it is possible to cast a second-preference vote in case a first-choice candidate is eliminated, voters are encouraged to vote sincerely for their preferred candidate, regardless of their chances of making the top two. This mitigates some of the tactical voting that commonly takes place in ‘First Past the Post’ elections, although voters may wish to cast a tactical second-preference vote based on who they think is likely to be among the top two candidates.
Wasted votes: As voters may only express two choices, it is possible that a large number of first-preference votes will be excluded from the second round of counting. If these voters opted for a second-preference candidate who has already been eliminated, or chose not to rank a second candidate, these votes are disregarded.
Tactical guesswork: SV quickly whittles down a potentially large field of candidates to a choice between two parties, so votes for smaller parties are often only considered significant for their second-preference votes. As SV requires voters to cast both votes at once, it is not always clear which candidates will advance to the
second round, potentially leading to a large number of wasted votes.
Absolute majority not guaranteed: Since voters do not have to cast a second preference and any second preference votes for a candidate who is not in the top two are disregarded, it is possible that the eventual winner fails to achieve a majority of overall voters. In London mayoral elections, the winner only won an absolute majority of votes (including non-transferable, or wasted, votes) in 2016.
Not a proportional voting system: Since winning candidates do not require an absolute majority of total votes cast, SV retains the problem of fairness and representation brought up with FPTP elections. Although this is not as big an issue for single-post mayoral elections, if SV were to be used for parliaments or councils, it could produce a large winner’s bonus for dominant parties, leaving smaller parties underrepresented.