The single non-transferable vote or SNTV is an electoral system used in multi-member constituency elections.
In any election, each voter casts one vote for one candidate in a multi-candidate race for multiple offices. Posts are filled by the candidates with the most votes. In a three seat constituency, the three candidates receiving the largest numbers of votes would win office.
There are three seats to be filled and five candidates: A, B, C, D and E.
C, D and E are the winning candidates.
This breaks down by party as:
Party Y got more votes than Party Z, but fewer seats because of an inefficient spread of votes across the candidates. If either party had risked trying to win all three seats, then Party X would have a higher chance of winning a seat, in the event of an uneven distribution of votes.
SNTV can result in proportional representation when political parties have accurate information about their relative levels of electoral support, and nominate candidates in accordance with their respective level of electoral support. If there are n candidates to be elected, Candidate A can guarantee being elected by receiving one more than 1/(n+1) of the votes (the Droop quota), because n other candidates cannot all receive more than Candidate A. It can become very difficult for parties to receive representation proportional to their strength, because they are forced to judge their strength prior to deciding how many candidates to field (strategic nomination). If they field too many, their supporters votes might be split across too many candidates, evenly diluting their share to the point where they all lose to a less diluted opposing party. If the party fields too few candidates, they might not win seats proportional to their hypothetical true level of support and excess votes would be wasted on their winning candidates.
The relative risks of strategic nomination are not the same for parties in other positions of electoral success. A large party with a majority of seats would have much more to lose from the split vote effect than to gain from avoiding the wasted vote effect, and so would likely decide to err on the side of fielding fewer candidates. A small party with little representation would be more risk-tolerant and err on the side of too many candidates, potentially gaining seats greater than their proportion of the electorate by winning with narrower margins of victory than the candidates from larger parties.
SNTV electoral systems, like proportional electoral systems generally, typically produce more proportional electoral outcomes as the size of the electoral districts (number of seats in each constituency) increases.
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