Voting system

Criteria in evaluating voting systems

In the real world, attitudes toward voting systems are highly influenced by the systems' impact on groups that one supports or opposes. This can make the objective comparison of voting systems difficult. In order to compare systems fairly and independently of political ideologies, voting theorists use voting system criteria, which define potentially desirable properties of voting systems mathematically.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It is impossible for one voting system to pass all criteria in common use. For example, Arrow's impossibility theorem demonstrates that several desirable features of voting systems are mutually contradictory. For this reason, someone implementing a voting system has to decide which criteria are important for the election.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Using criteria to compare systems does not make the comparison completely objective. For example, it is relatively easy to devise a criterion that is met by one's favorite voting method, and by very few other methods. Then one can make a biased argument in favor of the criterion, instead of directly in favor of the method. No one can be the ultimate authority on which criteria should be considered, but the following are some criteria that are accepted and considered to be desirable by many voting theorists:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• Majority criterion - Does the first choice of a majority win?
• Mutual majority criterion - If a majority prefers every choice in a set to all other choices, is a member of the set selected?
• Monotonicity criterion - Is it impossible to cause a choice to lose by ranking it higher, or win by ranking it lower?
• Consistency criterion - If the electorate is divided in two and a choice wins in both parts, does it win overall?
• Participation criterion - Is it always better to vote honestly than to not vote?
• Condorcet criterion - If a choice beats every other choice in pairwise comparison, does it win?
• Smith criterion - If every choice inside a set beats every choice outside the set in pairwise comparison, is a member of the set selected?
• Condorcet loser criterion - If a choice loses to every other choice in pairwise comparison, is it guaranteed not to win?
• Independence of irrelevant alternatives - Does the winner never change from A to B just because an unrelated choice C enters the race?
• Independence of clone candidates - If multiple similar choices are available, is the result of the election unaffected by their presence, or do they help or hurt each other?

Voting systems are also judged with criteria that are not mathematically precise but are still important, such as simplicity, speed of vote-counting, the potential for fraud or disputed results, strategic voting, and (for multiple-winner methods) the degree of proportionality.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~