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Supplementary Vote

The Supplementary Vote electoral system allows voters to cast a first and second preference vote on one ballot. If no candidate gains at least half of first preference votes, then second preference votes gained by the two leading candidates are taken into consideration. This is designed to ensure that the elected candidate has closer to majority support than would be the case under First-past-the-post. SV is used for Police and Crime Commissioner elections in England and Wales, and for the positions of directly-elected mayors in England, including London, Manchester, and Liverpool.

Example election

Taking the results of the 2016 Police and Crime Commissioner election in Nottinghamshire, the Labour candidate won less than 50% of the first preference vote, so a second round was needed. The second preference votes from the eliminated independent and UKIP candidates were counted, and were awarded to either Labour or Conservatives.

2016 Nottinghamshire Police and Crime Commissioner election

Party First Preference Votes First + Second Preference Votes
Labour 80,926 89,749 (Elected)
Conservative Party 48,155 56,105
UKIP 20,030 Eliminated
Independent 14,579 Eliminated
Independent 7,164 Eliminated
Total 170,854

As you can see, of the 41,773 votes cast for the three eliminated candidates, only 16,773 cast second preference votes for either Labour of Conservative. Some voters may have decided not to cast a second preference vote, or may have voted for one of the other eliminated candidates as a second preference.