# Voting system criterion

Wikipedia has an article on:
Wikipedia has an article on:
Wikipedia has an article on:
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A "voting system criterion" (or "electoral system criterion") is formally defined pass/fail criterion by which a voting system may be assessed.

## Criterion failure rates

Though a voting method may pass or fail a given criterion, that does not mean the voting method can't almost always pass or fail the criterion in practice, or that when it passes or fails the criterion, that this will be particularly bad. Advocates of various voting methods often make the argument that though their method may fail some criteria, that this should not be considered a major drawback of their methods; for example, advocates of Approval voting and IRV often argue that though those methods fail the Condorcet criterion, they almost always meet it in practice, and that when they fail it, it is for good reason, or at least not particularly bad.

### Efficiency

For some criteria, it is common to use the term "efficient" or "efficiency" to indicate that the criterion is always met by some voting method, or to identify how often that is the case. For example, Smith efficiency measures how often a voting method passes the Smith criterion.

## Sets

Many criteria relate to sets of candidates; see the set theory article for more information.

### Terminology note

Further, a common terminology when comparing two criteria is to say one is stronger than the other when it applies to every situation the other applies to and more (a superset), with weaker meaning it applies to only a subset of the situations.

## Relative importance of various criteria

### Essential criteria

Some criteria are very widely agreed to be important. Examples:

### Desirable criteria

Other criteria are also widely regarded as good, but there is disagreement over how important it is for a voting method to pass these (they are agreed to be desirable, but not necessarily essential):

Sometimes desirable properties or criteria are called desiderata.

There is disagreement over how important the various other criteria are. Some criteria are even considered bad by some; for instance, Michael Dummett, in a letter to Robert Newland, regarded the combination of later-no-harm and later-no-help as "quite unreasonable".[1]

## Examples

Examples for such criteria are:

### Majority-related criteria

Here are some criteria often touted by advocates of majority rule, split into categories of "widely agreed on" and criteria which are more polarizing:

### Strategic voting-related criteria

#### Protecting voters' preferences

These can generally be split into two categories: criteria which protect a voter from hurting a candidate (which doesn't prevent the voter themselves from getting hurt, since they may end up hurting a candidate not covered by the criterion), and criteria that protect a voter from hurting themselves.

### Mathematical criteria

#### Axiomatic criteria

These generally are considered essential and basic features of any voting method

## Types of criteria

### Absolute criterion

An absolute criterion requires or prohibits some result due to some characteristic of a given a set of ballots. This is in contrast to the below-mentioned relative criterion, which requires (or prohibits) a change in the election's result given some modification to the ballots.

Examples of absolute criteria:

### Relative criterion

A relative criterion requires that when the ballots are changed in some way, the result of the election must or must not change in some way. This is in contrast to the above-mentioned absolute criterion, which requires some result given some characteristic of a set of ballots.

Examples of relative criteria:

### Consensus criterion

Consensus criteria attempt to guarantee the election of consensus candidates. Examples of such criteria include greatest possible consensus criterion and unanimous consensus criterion.

## Notes

### Proportional Representation

Proportional representation is the general idea that groups of voters with shared preferences should be able to win an amount of representation in a multi-winner body (a legislature) proportional to how large they are. In partisan PR methods, proportionality can be measured using various measures of how well a party's seats matched up to its share of votes. For nonpartisan methods, there is disagreement on how to measure or quantify PR. Ranked PR advocates tend to tout Proportionality for Solid Coalitions, which is meant to account for coherent factions that can be identified from the rankings, while cardinal PR advocates gravitate towards the similar, but weaker, ￼￼Hare quota criterion and similar criteria.

Several criteria have rated-ballot or other adaptations that may make more sense in certain contexts. For example, the majority criterion says that a candidate preferred by a majority over all other candidates must win. The Majority criterion for rated ballots further requires the majority to give this candidate the highest score. It can be argued that a voter who gives their favorite candidate less than full support (i.e. didn't do normalization) doesn't deserve full power, so this modification to the criterion ensures that only a strategic or strongly supportive majority gets their way. Similar adaptations can be made to any criterion involving voter preferences determining who should win, such as PSC, the plurality criterion, Mutual majority, etc.