Proportional representation

Proportional Representation (PR) is a measure of the outcome of an election where there are multiple parties and multiple members are elected. It is one of many types of representation in a representative government.

In practice, the implementation involves ensuring that political parties in parliament or legislative assemblies receive a number of seats (approximately) proportional to the percentage of the vote they received by making use of a partisan system. One system which achieves high levels of proportional representation is party-list proportional representation. Another kind of electoral system strives to achieve proportional representation, but without relying on the existence of political parties. A common example of this is the single transferable vote (STV).

Measures

There are several metrics that are used to define Proportional Representation explicitly. A well-accepted form is the Gallagher index, which measures the difference between the percentage of votes each party gets and the percentage of seats each party gets in the resulting legislature, and aggregates across all parties to give a total measure in any one given election result. This measure attributes a specific level or Proportional Representation to a given election which can then be used in comparing various levels of proportionality among various elections from various Voting systems.

Michael Gallagher, who created the index, referred to it as a "least squares index", inspired by the residual sum of squares used in the method of least squares. The index is therefore commonly abbreviated as "LSq" even though the measured allocation is not necessarily a least squares fit. The Gallagher index is computed by taking the square root of half the sum of the squares of the difference between percent of votes ($V_{i}$) and percent of seats ($S_{i}$) for each of the political parties ($i=1,\ldots ,n$).

$\mathrm {LSq} ={\sqrt {{\frac {1}{2}}\sum _{i=1}^{n}(V_{i}-S_{i})^{2}}}$  Template:Sfn

The index weighs the deviations by their own value, creating a responsive index, ranging from 0 to 100. The larger the differences between the percentage of the votes and the percentage of seats summed over all parties, the larger the Gallagher index. The larger the index value, the larger the disproportionality, and vice versa. Michael Gallagher included "other" parties as a whole category, and Arend Lijphart modified it, excluding those parties. Unlike the well-known Loosemore–Hanby index, the Gallagher index is less sensitive to small discrepancies.

While the Gallagher index is considered the standard measure for Proportional Representation, Gallagher himself considered the Sainte-Laguë method "probably the soundest of all the measures." This is closely related to Pearson's chi-squared test which has better statistical underpinning.

$\mathrm {SLI} =\sum {(S-V)^{2} \over V}$

The failing of all such measures is the assumption that each vote is cast for one political party. This means that the only system which can be used is a Partisan system. Under the assumption that a plurality vote for a candidate represents a vote for their party, these measures can be applied to plurality voting systems like Single Member Plurality and Mixed Member Proportional. The consequence of this limitation is that Proportional Representation is not defined for systems without vote splitting. PSC is the most direct generalization.

It is worth noting that because there are disagreements on how best to conceptualize of PR, some measures look at how much each voter likes their favorite candidate i.e. the one meant to "represent them" (such as Monroe's method) while others look at how satisfied each voter is with all of the elected representatives.

Proportional Representation Criteria

Since the standard definitions of Proportional Representation do not apply to nearly all modern systems it has become common to define proportional representation in terms of passing some sort of criteria. There is no consensus on which criteria need to be passed for a parliament to be said to be proportional.

Proportionality for Solid Coalitions Criterion

If a sufficiently-sized group (generally at least a Droop or Hare quota) prefer a set of candidates above all others, do at least a proportional number (being the number of quotas the group comprises rounded down to the nearest integer) of candidates from that set (supposing there are enough of them) get elected?

Proportional (Ideological) Representation Criterion

Whenever a group of voters gives max support to their favoured candidates and min support to every other candidate, at least one seat less than the portion of seats in that district corresponding to the portion of seats that that group makes up is expected to be won by those candidates.

One of the effects of this property is that if all voters vote solely on party lines (max support to everyone in your party and min support to everyone outside of it), then the proportion of popular vote for candidates associated to parties is roughly equal to the proportion of members elected for each party. This is identical to “Partisan Proportionality” in the case that all groups large enough to expect a winning candidate have a party which they identify with and their candidate belongs to.

Partisan Proportionality Criterion

How similar are the proportion of the voters who support a party to the proportion of the parliament when voters deploy the strategy that maximizes the number of seats their preferred party gets (in most methods, this strategy is voting solely on party lines, i.e. max support to everyone in your party and min support to everyone outside of it)? This is a calculation for a specific outcome of a specific election. There are multiple different methods to be used but the most common is the Gallagher index. Specific systems can be judged under such metrics by the average expected value. This metric is nearly an exact restatement of the concept of Proportional Representation and as such, it cannot be defined in many cases.

Hare Quota Criterion

Whenever more than a Hare Quota of the voters gives max support to a single candidate and min support to every other candidate, that candidate is guaranteed to win regardless of how any of the other voters vote.

Any method that passes the Proportional Representation Criterion also passes the Hare Quota Criterion.

Winner Independent Proportionality Criterion

If at least n quotas of ballots approve the same set of candidates, but there is partial disagreement on m elected candidates outside of that set, then at least n-m candidates in the set must be elected. (If 2 quotas approve ABCD, 2 quotas approve ABCDE, and E is elected, the standard PR criterion would require 2 of ABCD to be elected, whereas this criterion would require 3 of ABCD to be elected.)

Combined Independent Proportionality Criterion

The winner set must be proportional even if some losing candidates were disqualified, scores for some losing candidates were reduced, and/or the scores for some winning candidates were increased. That is, if at least n quotas of ballots approve the same set of candidates, but there is partial disagreement on some candidates outside of that set, m of whom were elected, then at least n-m candidates in the set must be elected. (If 2 quotas approve ABCD, 2 quotas approve ABCDE, the standard PR criterion would require 2 of ABCD to be elected, whereas this criterion would require 4 of ABCDE to be elected.)

Proportional Systems

No system can be defined as giving exact proportional results unless a number of assumptions are made

1. The metric for proportionality must be defined and the winner selection defined under those terms
2. There is a clear relationship between the vote and the endorsement for a single party

This means that only Partisan Systems can be exactly proportional. Conversely, all systems have some level of Proportional Representation since metrics like Gallagher index never reach the maximum values. The criteria above are often used to define proportionality for modern systems like Sequentially Spent Score or Sequential proportional approval voting. The most common being Hare Quota Criterion. These are normally implemented as a number of multi-member districts that together form a parliament. Each district produces results guaranteed to pass the Hare Quota Criterion.

The district magnitude of a system (i.e. the number of seats in a constituency) plays a vital role in determining how proportional an electoral system can be. When using such systems, the greater the number of seats in a district or constituency, the more Proportional Representation it will achieve.

However, multiple-member districts do not need to use a system that passes any of these proportionality criteria. For example, a bloc vote would not pass any of the criteria.

An interesting quirk for implementation is that many Partisan Systems are altered in order to remove representation from groups. For example, in a Party List system it is common to add a threshold, that a party needs some percent of votes to receive any seats. The effect of this is that the major parties receive relatively equitable results but the fringe parties receive none.

Semi-proportional Systems

A "semi-proportional" system is made of several regional Multi-Member Districts with each passing some measure of Proportional Representation. While each district is in itself going to produce results with High Proportional Representation, the assembly as a whole will not. For larger parties, the results will tend to be fairly high in proportional representation because the variation from each district is averaged out over the group. For smaller parties, there is a threshold for entry so they may receive no seats. This is normally viewed as a positive feature since partisan systems often impose such a threshold to keep out small extremist groups.

Semi-Proportional systems can be constructed from any multi-winner system. However, they are typically done with sequential non-partisan systems, such as the single transferable vote and Reweighted score voting.

An alternative, perhaps more common definition of semi-proportional is that a voting method must pass some weak form of Proportionality for Solid Coalitions e.g. allowing voters to get PSC-like outcomes through strategic voting. Something like SNTV would classify as semi-proportional under this definition.

Advocacy

Proportional representation is unfamiliar to many citizens of the United States. The dominant system in former British colonies was single member plurality (SMP), but mixed-member proportional representation (MMP) and single transferable vote (STV) replaced it in a number of such places.

Systems designed to have high levels of Proportional representation do have some history in the United States. Many cities, including New York, once used such systems for their city councils as a way to break up the Democratic Party monopolies on elective office. In Cincinnati, Ohio, a system was adopted in 1925 to get rid of Republican party dominance but was successfully overturned in 1957.

Some electoral systems incorporate additional constraints on winner selection to ensure quotas based on based on gender or minority status (like ethnicity). Note that features such as this are not typically associated with "proportional representation" although the goal of such systems is to ensure that elected member representation is proportional to such population percentages. Many proportional representation advocates argue that, voters will already be justly represented without these demographic rules since the particular immutable characteristics are independent of partisan allegiance, ideology or ability as a politician.

Non-Partisan Definitions

In the case of non-partisan voting, the definition of proportional Representation is undefined. Metrics like Gallagher index can no longer be defined. For non-partisan multi-member systems, for ranked methods, there is generally one minimum requirement for proportionality, Proportionality for Solid Coalitions (though see the Monroe's method article for an alternative idea), while for cardinal PR methods, there are four main competing philosophies between what is and is not proportional: Phragmén, Monroe, Thiele and Unitary.

• Proportionality for Solid Coalitions (PSC) requires that when quotas of voters prefer sets of candidates above all others, a proportional number of candidates from those sets must be elected.
• Under the Phragmén interpretation, voting is a distribution problem where the representation weight of candidates must be fairly spread across the different voters to produce the most equitable representation possible. The winner set composed of candidates which best distribute the candidates representation is the most proportional.
• Under the Monroe interpretation, voting is an attribution problem where every candidate has a quota of voters to be filled with specific voters. The winner set composed of candidates which maximizes the sum of score for the voters in that candidate’s quota is the most proportional. The voting method is impartial to how anybody outside of that candidate’s quota rates them.
• Under the Thiele interpretation, voters have vote weight which should be distributed across candidates. The proportion of ballot weight assigned to each winner is the amount which that candidate supports their election. Under this interpretation, the more an outcome maximizes the sum of all score when reweighted by ballot weight, the more proportional it is.
• Under the Unitary interpretation interpretation of each voter has an fixed amount of utility to be spent on candidates. When a candidate is elected their power to elect subsequent candidates is lower directly proportionally to the amount of utility previously spend on prior candidates. This interpretation can be thought of as an additional constraint on the Monroe interpretation but since the philosophy is about voters spending points on candidates rather than voters themselves being assigned to candidates it is a distinct interpretation of proportional representation. The Unitary interpretation is in some way the inverse interpretation of the Phragmén interpretation. In the former each voter has a conserved amount of vote weight to spend on candidates and in the latter the each candidate has a conserved amount of representation weight to distribute over the voters.

Example Systems

System	Philosophy	Comment
Single transferable vote	PSC or Monroe interpretation	Ordinal ballots
Sequential Monroe voting	Monroe interpretation	-
Sequentially Spent Score	Unitary interpretation	-
Sequentially Shrinking Quota	Unitary interpretation	May not be strictly Unitary but follows from the theory
Sequential proportional approval voting	Thiele Interpretation	Approval ballots only
Reweighted Range Voting	Thiele Interpretation	May not be strictly Thiele but follows from the theory
Single distributed vote	Thiele Interpretation	A more Thiele implementation of Reweighted Range Voting
Sequential Phragmen	Phragmén interpretation
Sequential Ebert	Phragmén interpretation

The backstory

Thiele, a Danish statistician, and Phragmen, a mathematician have been debating these two philosophies in Sweden. Thiele originally proposed Sequential Proportional Approval Voting in 1900 and it was adopted in Sweden in 1909 before Sweden switched to Party List voting afterward. Phragmen believed there were flaws in Thiele’s method, and came up with his own sequential method to correct these flaws, and that started a debate about what was the ideal metric of proportionality. Thiele also came up with the approval ballot version of harmonic voting, however during that time the harmonic method was too computationally exhaustive to be used in a governmental election. Both his sequential proportional approval voting and his approval ballot version of the harmonic method was lost to history until about a century later when they were independently rediscovered.

The Monroe interpretation named after the first first person to formalize the concept, Burt Monroe. Single transferable vote is a Monroe type system which predates this formalization so it is clear that the core idea had existed for some time.

Keith Edmonds saw a unification of Proportional Representation and the concept of one person one vote which was maintained throughout winner the winner selection method. He coined the term "vote unitarity" for the second concept and designed a score reweighting system which satisfied both Hare Quota Criterion and Vote Unitarity. As such it would preserve the amount of score used through sequential rounds while attributing representation in a partitioned way similar to Monroe. It would assign Hare Quotas of score to winners which allowed for a voters influence to be spread over multiple winners as opposed to Monroe which assigns a whole ballot with no spreading. Since score is a conserved quantity which is spent like money there is a natural analogy to Market based voting. This concept was heavily influence by economic theory not the Monroe interpretation even though the resultant mathematical formulation is quite similar.

Comparison

Proportionality for Solid Coalitions is praised for ensuring that voters get what would intuitively be considered an at least somewhat proportional outcome, but is criticized for focusing too much on giving a voter one "best" representative, rather than letting that voter have influence in electing several representatives.

Many of the properties of these systems can be derived from their party list simplifications. The Balinski–Young theorem implies that not all desirable properties are possible in the same system. Theile type systems reduce to divisor methods which means that adding voters or winners will not change results in undesirable ways. The other three reduce to Largest remainder methods which obey Quota Rules but adding voters or winners may change outcomes in undesirable ways. One such way is failure of Participation criterion. It is not clear which is a fundamentally better choice since Quota Rules are inanimately tied with some definitions of proportionality.

Phragmen and Monroe share many desirable and undesirable properties. Most importantly a lack of convexity, the ability for votes that give every candidate the same score to affect the outcome. There are also election scenarios where both philosophies pick what is clearly the wrong winner. Further details can be found in the “Pereira’s Complaints about Monroe” section of Monroe’s method or the “Major defect pointed out by Toby Pereira” section of this Phragmen-Type method)

However neither fail the Universally liked candidate criterion which is a criterion that Thiele type methods fail.

Benefits of the Phragmen/Monroe/Unitary measure of proportionality:

Passes the ULC criteria. For Thiele-type methods, because they fail ULC, every time a candidate that every voter gave a max rating to wins, the distribution of the remaining winners becomes more majoritarian/utilitarian.

Benefits of the Thiele measure of proportionality:

Adding ballots that give every candidate the same score can’t change which outcome is considered the best. Convexity. Warren's multi-winner participation criteria.

Criticisms of the Phragmen metric:

Taken to its limits, Phragmen-thinking would say, once the 50% Reds elected a red MP, and the 50% Blues elected a blue MP, there was no benefit whatever to replacing the red MP by somebody approved by the entire populace.

Criticisms of the Thiele metric:

The Universally liked candidate criterion can be exemplified with the following example. Three people share a house and two prefer apples and one prefers oranges. One of the apple-preferrers does the shopping and buys three pieces of fruit. But instead of buying two apples and an orange, he buys three apples. Why? Because they all have tap water available to them already and he took this into account in the proportional calculations. And his reasoning was that the larger faction (of two) should have twice as much as the smaller faction (of one) when everything is taken into account, not just the variables. Taken to its logical conclusion, Thiele-thinking would always award the largest faction everything because there is so much that we all share – air, water, public areas, etc!

The trouble with this is, politicians are not like tap water and oranges. That reasoning would make sense if politicians were “wholly owned” by the Blues, just as Peter wholly-eats an apple. But even the most partisan politicians in Canada do a lot of work to help Joe Average constituent whose political leanings they do not even know. At least, so I am told.

Pick your poison: it seems that all proportional voting methods must fail one of two closely related properties:

If a group of voters gives all the candidates the same score, that cannot affect the election results (ex: if you gave every candidate a max score, your vote shouldn’t change who is and isn’t a winner any more so than you would change the results by just not voting).

If some of the winners are given the same score by all voters, that cannot affect the proportionality of the election results among the remaining winners (ex: if you removed a candidate that is given a max score by all voters, and ran the election again such that you were electing 1 less winner, the only difference between that election result and the original election result should be that it does not contain the universally liked candidate).

Phragmen/Monroe-type methods fail 1. and Thiele-type methods fail 2. and as of this point, it doesn’t seem possible to have them both without giving up PR.

Alternatives

Due to the ambiguity and difficulty in the definition of Proportional Representation academic work often uses another more robust metric. This is the concept of a Stable Winner Set. The requirement that a system always produces a stable winner set when there exists one is definable in all possible systems. This makes it more useful than the concept of Proportional Representation which is typically tied to Partisan voting and as such cannot be defined for all systems. This concept evolved out of the economics field of Participatory Budgeting but can be equally suitable in Social Choice Theory. A less strict and more practical version of this is Justified representation.

Notes

Party list case

The party list case of a proportional voting method is what type of Party list allocation method it becomes equivalent to when voters vote in a "Party list"-like manner (i.e. they give maximal support to some candidates and no support to all others, as if voting on party lines). Generally, the party list case of a PR method will either be a divisor method, such as D'Hondt, or a Largest remainder method, such as Hamilton. PR methods can generally be split into two categories: sequential (one winner is elected at a time) and optimal (every possible winner set is compared to each other and the best one is chosen).

Almost all sequential PR methods can have a single-winner method done to elect the final seat; this is because at that point there is only one seat left to elect. See Single transferable vote#Deciding the election of the final seat for an example. Condorcet methods and STAR voting can be made to work with PR methods in this way.

See the combinatorics article for more information.

See Also

• Vote splitting
• Proportionate Representation
• Ideal Representation
• Types of representation

Further reading

• John Hickman and Chris Little. "Seat/Vote Proportionality in Romanian and Spanish Parliamentary Elections" Journal of Southern Europe and the Balkans Vol. 2, No. 2, November 2000safd
• See the Proportional Representation Library (created by Professor Douglas J. Amy, Mount Holyoke College and now maintained by FairVote):
  • FairVote
  • Mount Holyoke College

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