Sri Lankan Supplementary Vote
In Sri Lanka a system that may be considered a variant of the Supplementary Vote (SV) is used to elect the president. Under the Sri Lankan form of the Supplementary Vote (Sri Lankan SV) each voter ranks from among the list of candidates a first, a second and a third preference. If no candidate receives an absolute majority of votes on the first count all but the two leading candidates are eliminated and the second and third preferences of those who voted for eliminated candidates are redistributed to help determine a winner.
The current Sri Lankan presidential voting system is sometimes described as a form of the Supplementary Vote and sometimes as a variant of Instant Run-off Voting, but differs from both in important respects. It is identical to the Supplementary Vote except that voters are permitted to express three preferences rather than just two. It differs from Instant Run-off Voting in this limitation on preferences but also in that there are only two rounds of counting. It can also be considered a variant of Run-off Voting, in which both 'rounds' occur without the need for a second round.
Each voter ranks at least one and no more than three candidates in order of preference. E.g.
Counting the votes
In the first round, if a candidate has the support of an absolute majority (i.e. more than half) of the total number of first preferences expressed they are deemed elected. If no candidate has such a majority then all candidates except the two who have received the largest number of first preferences are eliminated and the count proceeds to a second round.
In the second round any voter whose first preference has been eliminated has their vote transferred to the candidate of their second preference, but only if their second choice has not also been eliminated. If there second choice has also been eliminated then their vote is transferred to their third choice (provided that that candidate has also not been eliminated). The candidate with the most votes is then declared elected.
Imagine that Tennessee is having an election on the location of its capital. The population of Tennessee is concentrated around its four major cities, which are spread throughout the state. For this example, suppose that the entire electorate lives in these four cities, and that everyone wants to live as near the capital as possible.
The candidates for the capital are:
- Memphis on Wikipedia, the state's largest city, with 42% of the voters, but located far from the other cities
- Nashville on Wikipedia, with 26% of the voters, near the center of Tennessee
- Knoxville on Wikipedia, with 17% of the voters
- Chattanooga on Wikipedia, with 15% of the voters
The preferences of the voters would be divided like this:
| 42% of voters
(close to Memphis)
| 26% of voters
(close to Nashville)
| 15% of voters
(close to Chattanooga)
| 17% of voters|
(close to Knoxville)
Assuming each voter votes according to their sincere preferences (for a more sophisticated approach, see below), Nashville and Memphis would receive the most votes and advance to the second round.
The second preference of voters from Chattanooga is for Knoxville. However Knoxville has been eliminated so the votes must be transferred to the third choice of Chattanooga voters; this is Nashville, which remains in the race. The second preference of voters from Knoxville is for Chattanooga. Chattanooga has been eliminated so their votes also transfer to their third choice; again this is Nashville.
On the second and final count, therefore, all the votes from the two eliminated candidates transfer to Nashville. Nashville now has more votes than Memphis and so Nashville is declared the winner. Note that under 'conventional' SV the winner would have been Memphis.
Potential for tactical voting
Under Sri Lankan SV, unlike under Instant Run-off Voting, a voter will not influence the final result of an election unless they express either a first, a second or a third preference for one of the two leading candidates. Furthermore, as under 'conventional' SV, their first preference will not be of influence unless it is expressed for one of the three leading candidates.
Like conventional SV, Sri Lankan SV is vulnerable to the tactics of 'push over' and 'compromise'. Compromise occurs when a voter insincerely ranks a candidate higher than their true order of preference in order to secure a more favourable election result. SV is less vulnerable to the tactic of 'compromise' than the Single Member Plurality ('First-Past-the-Post') system but more so than Instant Run-off Voting. Push-over is the tactic used where voters who prefer one of the two leading candidates in an election help their favourite candidate by expressing a first preference for a less popular third candidate in order to bring it about that their favourite candidate faces a weaker rival than their actual strongest opponent when they advance to the final round.
Both Sri Lankan SV and conventional SV fail the monotonicity criterion. This means that sometimes it is paradoxically possible both to harm the chances of a candidate by ranking them higher, and to aid the chances of a candidate by ranking them lower.
Impact on factions and candidates
The Sri Lanka SV, like conventional SV, is not a form of proportional representation, and were it used to elect a council or legislature, it could be expected to over-represent larger parties at the expense of smaller ones in the same manner as other systems based on single winner elections, such as Single Member Plurality and Instant Run-off Voting.
All forms of the Supplementary Vote are said to encourage candidates to seek support beyond their core base of supporters in order to secure the lower preferences of the supporters of other candidates. This is said to create a more conciliatory campaigning style among candidates with similar policy platforms. SV also improves the chances of 'third party' candidates by encouraging voters who wish to do so to vote sincerely for such candidates where under systems such as the Single Member Plurality system they would be discouraged from doing so for tactical reasons.
Third party candidates are especially advantaged under the Sri Lankan form of SV because it permits three preferences rather than just two. This means that a voter might express preferences for two third party candidates while still granting their third choice to one of the two largest parties. Under 'conventional' SV someone voting in the same manner could only vote for one third party candidate.
As with conventional SV, the potential advantages of Sri Lankan SV in terms of encouraging third party candidates and conciliatory campaigning may be greatly qualified by the strong incentives it creates for tactical voting.
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