Difference between revisions of "Range voting"
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[[Category:Single-winner voting ]]
Latest revision as of 08:33, 11 May 2017
Range voting, or ratings summation, or average rating, or cardinal ratings is a voting system used for single-seat elections. (It can also be used for multi-seat elections, but not with proportional results.) It is also used on the web - for rating movies (Internet Movie Database), comments (Kuro5hin), and many other things - and something very similar to it is used in the Olympics to award gold medals to gymnasts.
Range voting uses a ratings ballot; that is, each voter rates each candidate with a number. In "pure numerical voting," each voter may give any candidate any real number (i.e. not restricted to any finite range), but as the potential for tactical voting would then be huge, most systems use upper and lower bounds. For example, each voter might give a real number between -1 and 1, or between 0 and 99; in the latter case little is lost by also demanding that the scores be integers.
Range voting in which only two different votes may be submitted (0 and 1, for example) is equivalent to approval voting. In range (or approval) voting with blanks, the voter is allowed to leave some scores blank to denote ignorance about those candidates.
Range voting satisfies the monotonicity criterion, the participation criterion, the Consistency Criterion, the summability criterion, the Favorite Betrayal criterion, Independence of irrelevant alternatives, the Non-compulsory support criterion and independence of clones.
Range voting does not comply with the Condorcet criterion because it allows for the difference between 'rankings' to matter. E.g. 51 people might rate A at 100, and B at 90, while 49 people rate A at 0, and B at 100. Condorcet would consider this 51 people voting A>B, and 49 voting B>A, and A would win. Range voting would see this as A having support of 5100/100 = 51%, and B support of (51*90+49*100)/100 = 94.9%; range voting advocates would probably say that in this case the Condorcet winner is not the socially ideal winner.
Counting the Votes
The scores for each candidate are summed, and the candidate with the highest sum is declared the winner. In range voting with blanks the candidate with the highest average score (where only nonblank scores are incorporated into the average) is the winner.
(Another method of counting is to find the median score of each candidate, and elect the candidate with the highest median score - see Median Ratings. Because strategic voting will typically lead to a vast number of candidates with the same median, a secondary measure to resolve ties is needed.)
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)
Suppose that voters were told to grant 1 to 4 points to each city, giving their most favorite 4 points, second favorite 3 points, third favorite 2 points, and least favorite 1 point. For simplicity, letâ€™s say we had 42 voters from Memphis, 26 from Nashville, 15 from Chattanooga, and 17 from Knoxville. The votes would be as follows.
|Memphis||42 * 4 = 168||26 * 1 = 26||15 * 1 = 26||17 * 1 = 26||226|
|Nashville||42 * 3 = 126||26 * 4 = 104||15 * 2 = 30||17 * 2 = 34||294|
|Chattanooga||42 * 2 = 84||26 * 3 = 78||15 * 4 = 60||17 * 3 = 51||273|
|Knoxville||42 * 1 = 42||26 * 2 = 52||15 * 3 = 45||17 * 4 = 68||207|
In general, the optimal strategy for range voting is to vote it identically to approval voting, so that all candidates are given either the maximum score or the minimum score. For more detailed strategies, see approval voting.
Range voting has an advantage over approval voting if voters are actually expressing their personal feelings rather than doing everything they can to cause their most favored outcomes.
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