STAR voting

From Electowiki
Jump to: navigation, search
File:Completed Score Voting Ballot version2.png
On a STAR voting ballot, the voter scores all the candidates

Template:Electoral systems STAR voting<ref>https://www.starvoting.us/</ref><ref>Template:Cite news</ref> is an electoral system for single-seat elections. The name stands for "score then automatic runoff", referring to the fact that this system is a combination of score voting to pick two frontrunners with the highest total scores, followed by a "virtual runoff" in which the frontrunner who is preferred on more ballots wins. It is a type of cardinal voting electoral system.

Usage

Political use

The concept was first proposed publicly in October 2014 by Center for Election Science co-founder Clay Shentrup.<ref>https://groups.google.com/d/msg/electionscience/JK82EFn7nrs/Lble3V2CW4UJ</ref> The runoff step was introduced in order to correct for strategic distortion in ordinary score voting,<ref>https://ivn.us/2016/12/08/score-runoff-voting/</ref> such as Bullet voting and tactical maximization.<ref>Template:Cite news</ref> Thus, STAR is intended to be a compromise between score voting and instant runoff voting.<ref>Template:Cite news</ref> The movement to implement STAR voting was centered in Oregon, and in July 2018, supporters submitted over 16000 signatures for a ballot initiative in Lane County, OR. This is enough to qualify this proposal to be on the ballot, if the measure is deemed well-drafted. <ref>Template:Cite news</ref>

Types

STAR voting uses a ratings ballot; that is, each voter rates each candidate with a number within a specified range, such as 0 to 5 stars<ref>http://rangevoting.org/RateScaleResearch.html</ref>. In the simplest system, all candidates must be rated. The scores for each candidate are then summed, and the two candidates with the highest sums go to the runoff. Of these two, the one that is higher on a greater number of ballots is the winner.

File:Score Runoff Voting sample presidential ballot.png
"Score Runoff Voting", an old name for STAR voting, uses a standard Score Voting ballot. The counting method adds an extra step to yield the preference winner between the top two scoring candidates overall.

Example

Tennessee's four cities are spread throughout the state

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)
  1. Memphis
  2. Nashville
  3. Chattanooga
  4. Knoxville
  1. Nashville
  2. Chattanooga
  3. Knoxville
  4. Memphis
  1. Chattanooga
  2. Knoxville
  3. Nashville
  4. Memphis
  1. Knoxville
  2. Chattanooga
  3. Nashville
  4. Memphis

Suppose that 100 voters each decided to grant from 0 to 5 stars to each city such that their most liked choice got 5 stars, and least liked choice got 0 stars, with the intermediate choices getting an amount proportional to their relative distance.

Voter from/
City Choice
Memphis Nashville Chattanooga Knoxville Total
Memphis 210 (42 × 5) 0 (26 × 0) 0 (15 × 0) 0 (17 × 0) 210
Nashville 84 (42 × 2) 130 (26 × 5) 45 (15 × 3) 34 (17 × 2) 293
Chattanooga 42 (42 × 1) 52 (26 × 2) 75 (15 × 5) 68 (17 × 4) 237
Knoxville 0 (42 × 0) 26 (26 × 1) 45 (15 × 3) 85 (17 × 5) 156

The frontrunners are Nashville and Chattanooga. Of the two, Nashville is preferred by 68% (42+26) to 32% (15+17), so Nashville, the capital in real life, likewise wins in the example. In this particular case, there is no way for any single city of voters to get a better outcome through tactical voting. However, Chatanooga and Knoxville voters combined could vote strategically to make Chatanooga win; while Memphis voters could defend against that strategy and ensure Nashville still won by strategically giving Nashville a higher rating.

For comparison, note that traditional first-past-the-post would elect Memphis, even though most citizens consider it the worst choice, because 42% is larger than any other single city. Instant-runoff voting would elect the 2nd-worst choice (Knoxville), because the central candidates would be eliminated early. In Approval voting, with each voter selecting their top two cities, Nashville would win because of the significant boost from Memphis residents. A two-round system would have a runoff between Memphis and Nashville where Nashville would win.

Properties

STAR voting allows voters to express preferences of varying strengths.

STAR voting satisfies the monotonicity criterion, i.e. raising your vote's score for a candidate can never hurt their chances of winning, and lowering it can never help their chances.

In summary, STAR voting satisfies the monotonicity criterion, the resolvability criterion, the majority criterion, and reversal symmetry. It does not satisfy either the Condorcet criterion (i.e., is not a Condorcet method) or the Condorcet loser criterion, although with all-strategic voters and perfect information the Condorcet winner is a Nash equilibrium.<ref>Laslier, J.-F. (2006) "Strategic approval voting in a large electorate," IDEP Working Papers No. 405 (Marseille, France: Institut D'Economie Publique)</ref> It does not satisfy the later-no-harm criterion, meaning that giving a positive rating to a less preferred candidate can cause a more preferred candidate to lose.

See also

Notes

Template:Reflist

External links

Template:Voting methods