The Supplementary Vote (SV) is a shortened version of the Alternative Vote (AV), involving either one or two rounds of counting. The SV system is employed to elect single office-holders for a whole regional or local authority area. SV has been successfully used to elect the London Mayor since 2000 and all the other directly elected mayors in England subsequently. In 2012 it was employed for the first time to elect Police Commissioners in England and Wales.

Under SV, unlike AV, voters are limited to a first and second preference choice. Normally there are two columns on the ballot paper – one for voters to mark their first choice and one in which to mark a second choice. Voters mark one ‘X’ in each column, although voters are not required to make a second choice if they do not wish to. All the first choices are then counted, and if a candidate receives more than 50 per cent of the first preference votes on the first count then they are elected.

If no candidate receives a majority, the top two candidates continue to a second round and all other candidates are set aside. The second-choice votes of everyone whose first choice has been set aside are then counted. Any votes for the two remaining candidates are then added to their first-round totals. Whichever candidate has the most votes after these second-preferences have been allocated is declared the winner.

This process of eliminating low ranked candidates and redistributing their voters’ second choices ensures that the largest feasible number of votes count in deciding who is elected. It does not always completely guarantee that the person elected has a majority of votes cast. However, in repeated London elections it’s worth noting that the winning mayor has had nearly three fifths support amongst votes counted.

The benefits of SV

  • SV encourages strategic campaigning, as not only first choice but also second choice votes are important.
  • SV gives the voter more power because both first and second preferences may count.
  • SV is a simpler system to understand compared to AV.

The disadvantages of SV

  • SV can promote voting for candidates from the main parties only.
  • The winning candidate need not have the support of at least 50 per cent of the electorate under SV
  • If voters think there will be no clear winner after round 1, they must guess which two will make the final round and allocate their choices accordingly. It is possible for voters to inadvertently defeat their preferred candidate.
  • By the nature of SV there can be a lot of wasted votes; because the votes cast in the first round may end up not being transferring and notcounted in the second round
  • Under the SV system there is a likelihood of tactical voting.

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