NAME

Vote::Count::TieBreaker

VERSION 1.213

Synopsis

  my $Election = Vote::Count->new(
    BallotSet      => $ballotsirvtie2,
    TieBreakMethod => 'approval'
  );

Tie Breakers

The most important thing for a Tie Breaker to do is it should use some reproducible difference in the Ballots to pick a winner from a Tie. The next thing it should do is make sense. Finally, the ideal Tie Breaker will resolve when there is any difference to be found. Arguably the best use of Borda Count is as a Tie Breaker, First Choice votes and Approval are also other great choices.

TieBreakMethod is specified as an argument to Vote::Count->new(). The TieBreaker is called internally from the resolution method via the TieBreaker function, which requires the caller to pass its TieBreakMethod.

TieBreakMethod argument to Vote::Count->new

  'approval'
  'all' [ eliminate all tied choices ]
  'borda' [ applies Borda Count to current Active set ]
  'grandjunction' [ more resolveable than simple TopCount would be ]
  'none' [ eliminate no choices ]
  'precedence' [ requires also setting PrecedenceFile ]

Grand Junction

The Grand Junction (also known as Bucklin) method is one of the simplest and easiest to Hand Count RCV resolution methods. Other than that it is generally not considered a good method.

Because it is simple, and always resolves, except when ballots are perfectly matched up, it is a great TieBreaker. It is not Later Harm Safe, but heavily favors higher rankings. It is the Vote::Count author's preferred Tie-Breaker.

The (Standard) Grand Junction Method

Only the Tie-Breaker variant is currently implemented in Vote::Count.

  1. Count the Ballots to determine the quota for a majority.

  2. Count the first choices and elect a choice which has a majority.

  3. If there is no winner add the second choices to the totals and elect the choice which has a majority (or the most votes if more than one choice reaches a majority).

  4. Keep adding the next rank to the totals until either there is a winner or all ballots are exhausted.

  5. When all ballots are exhausted the choice with the highest total wins.

As a Tie Breaker

The Tie Breaker Method is modified.

Instead of Majority, any choice with a current total less than another is eliminated. This allows resolution of any number of choices in a tie.

The winner is the last choice remaining.

TieBreakerGrandJunction

  my $resolve = $Election->TieBreakerGrandJunction( $choice1, $choice2 [ $choice3 ... ]  );
  if ( $resolve->{'winner'}) { say "Tie Winner is $resolve->{'winner'}"}
  elsif ( $resolve->{'tie'}) {
    my @tied = $resolve->{'tied'}->@*;
    say "Still tied between @tied."
  }

The Tie Breaking will be logged to the verbose log, any number of tied choices may be provided.

TieBreaker

Implements some basic methods for resolving ties. The default value for IRV is 'all', and the default value for Matrix is 'none'. 'all' is inappropriate for Matrix, and 'none' is inappropriate for IRV.

  my @keep = $Election->TieBreaker( $tiebreaker, $active, @tiedchoices );

TieBreaker returns a list containing the winner, if the method is 'all' the list is empty, if 'none' the original @tiedchoices list is returned. If the TieBreaker is a tie there will be multiple elements.

Precedence

Since many existing Elections Rules call for Random, and Vote::Count does not accept Random as the result will be different bewtween runs, Precedence allows the Administrators of an election to randomly or arbitrarily determine who will win ties before running Vote::Count.

The Precedence list takes the choices of the election one per line. Choices defeat any choice lower than them in the list. When Precedence is used an additional attribute must be specified for the Precedence List.

 my $Election = Vote::Count->new(
   BallotSet => read_ballots('somefile'),
   TieBreakMethod => 'precedence',
   PrecedenceFile => '/path/to/precedencefile');

CreatePrecedenceRandom

Creates a Predictable Psuedo Random Precedence file, and returns the list. Randomizes the choices using the number of ballots as the Random Seed for Perl's built in rand() function. For any given Ballot File, it will always return the same list. If the precedence filename argument is not given it defaults to '/tmp/precedence.txt'. This is the best solution to use where the Rules call for Random, in a large election the number of ballots cast will be sufficiently random, while anyone with access to Perl can reproduce the Precedence file.

  my @precedence = Vote::Count->new( BallotSet => read_ballots('somefile') )
    ->CreatePrecedenceRandom( '/tmp/precedence.txt');

TieBreakerFallBackPrecedence

This optional argument enables or disables using precedence as a fallback, generates /tmp/precedence.txt if no PrecedenceFile is specified. Default is off.

UntieList

Sort a list in an order determined by a TieBreaker method, sorted in Descending Order. The TieBreaker must be a method that returns a RankCount object, Borda, TopCount, and Approval, Precedence. To guarrantee reliable resolution Precedence must be used or have been set for fallback.

  my @orderedlosers = $Election->UntieList( 'Approval', @unorderedlosers );

UntieActive

Produces a precedence list of all the active choices in the election. Takes a first and optional second method name, if one of the methods is not Precedence, TieBreakerPrecedence must be true. The methods may be TopCount, Approval, or any other method that returns a RankCount object. Returns a RankCount object (with the OrderedList method enabled).

  my $precedenceRankCount = $Election->UntieActive( 'TopCount', 'Approval');

BUG TRACKER

https://github.com/brainbuz/Vote-Count/issues

AUTHOR

John Karr (BRAINBUZ) brainbuz@cpan.org

CONTRIBUTORS

Copyright 2019-2021 by John Karr (BRAINBUZ) brainbuz@cpan.org.

LICENSE

This module is released under the GNU Public License Version 3. See license file for details. For more information on this license visit http://fsf.org.

SUPPORT

This software is provided as is, per the terms of the GNU Public License. Professional support and customisation services are available from the author.