http://wiki.electorama.com/wiki/history/Monotonicity?feed=atomMonotonicity - Revision history2020-07-02T10:02:56ZRevision history for this page on the wikiMediaWiki 1.27.3http://wiki.electorama.com/w/index.php?title=Monotonicity&diff=69756&oldid=prevHomunq: Created page with " The '''monotonicity criterion''' is a voting system criterion used to evaluate both single and multiple winner ranked voting systems. A ranked voting system is '''mon..."2016-10-27T17:10:59Z<p>Created page with " The '''monotonicity criterion''' is a <a href="/wiki/Voting_system_criterion" title="Voting system criterion">voting system criterion</a> used to evaluate both single and multiple winner <a href="/wiki/edit/Ranked_voting_system?redlink=1" class="new" title="Ranked voting system (page does not exist)">ranked voting systems</a>. A ranked voting system is '''mon..."</p>
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The '''monotonicity criterion''' is a [[voting system criterion]] used to evaluate both single and multiple winner [[ranked voting system]]s. A ranked voting system is '''monotonic''' if it is neither possible to prevent the election of a candidate by ranking them higher on some of the ballots, nor possible to elect an otherwise unelected candidate by ranking them lower on some of the ballots (while nothing else is altered on any ballot).<br />
<ref name="Woodall-Monotonicity">D R Woodall, [http://www.votingmatters.org.uk/ISSUE6/P4.HTM "Monotonicity and Single-Seat Election Rules"], ''[[Voting matters]]'', Issue 6, 1996</ref> In single winner elections that is to say no winner is harmed by up-ranking and no loser can win by down-ranking. Douglas R. Woodall called the criterion '''mono-raise'''.<br />
<br />
Raising a candidate {{math|''x''}} on some ballots ''while changing'' the orders of other candidates does ''not'' constitute a failure of monotonicity. E.g., harming candidate {{math|''x''}} by changing some ballots from {{math|''z'' > ''x'' > ''y''}} to {{math|''x'' > ''y'' > ''z''}} isn't a violation of the monotonicity criterion.<br />
<br />
The monotonicity criterion renders the intuition that there should be neither need to worry about harming a candidate by (nothing else than) up-ranking nor it should be possible to support a candidate by (nothing else than) counter-intuitively down-ranking. <!--<br />
<br />
(Something similar (down-ranking CDU '''and''' additionally up-ranking another party, e.g. FDP) happened in the German federal election of 2005, in which conservative voters in Dresden deliberately voted against their party of choice (the CDU) in order to maximize that party's number of seats in the federal parliament. This was possible due to Germany's voting system (mixed member proportional with overhang seats computed independently for each federal state) and the fact that the vote in Dresden took place a week after the rest of the country due to the death of a candidate, enabling voters in Dresden to vote tactically in full knowledge of the results already achieved elsewhere. As a result of this, the German Constitutional Court ruled on July 3 2008 that the German voting system must be reformed to eliminate its non-monotonicity.<ref> See e.g. [http://fruitsandvotes.com/blog/?p=117]</ref><br />
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-->There are several variations of that criterion; e.g., what Douglas R. Woodall called ''mono-add-plump'': A candidate {{math|''x''}} should not be harmed if further ballots are added that have {{math|''x''}} top with no second choice. Agreement with such rather special properties is the best any ranked voting system may fulfill: The [[Gibbard–Satterthwaite theorem]] shows, that any meaningful ranked voting system is susceptible to some kind of [[tactical voting]], and [[Arrow's impossibility theorem]] shows that individual rankings can't be meaningfully translated into a community-wide ranking where the order of candidates {{math|''x''}} and {{math|''y''}} is always [[Independence of irrelevant alternatives|independent of irrelevant alternatives]] {{math|''z''}}.<!--<br />
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The result of David Austen-Smith and Jeffrey Banks that monotonicity in individual preferences is impossible is a nonissue: For given voter preferences v=v_1...v_n and a winner x under voting scheme alpha, they investigate changes in v, where e.g. altering v_i from a,b,c,d,x to d,c,x,b,a is allowed, which can't be seriously named a monotonicity property. That allows random permutations even ''ahead'' of x, and is therefore even more rigid than Woodall's mono-raise-random, which is already incompatible with [majority AND later-no-help AND later-no-harm].<ref>''[http://www.jstor.org/stable/1963173 Monotonicity in Electoral Systems]''. The American Political Science Review Vol. 85, No. 2, June 1991; p. 533.</ref><br />
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--> Noncompliance with the monotonicity criterion doesn't tell anything about the likelihood of monotonicity violations, failing in one of a million possible elections would be as well a violation as missing the criterion in any possible election.<br />
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Of the single-winner ranked voting systems, [[Borda count|Borda]], [[Schulze method|Schulze]], [[ranked pairs]], maximize affirmed majorities, descending solid coalitions,<ref>[http://wiki.electorama.com/wiki/Descending_Solid_Coalitions Electorama:Descending Solid Coalitions].</ref> and descending acquiescing coalitions<ref name="Woodall-Monotonicity" /><ref>[http://wiki.electorama.com/wiki/Descending_Acquiescing_Coalitions Electorama:Descending Acquiescing Coalitions].</ref> are monotonic, while [[Coombs' method]], [[runoff voting]], and [[instant-runoff voting]] (IRV) are not.<br />
<br />
Most variants of the [[single transferable vote]] (STV) [[proportional representation]]s are not monotonic, especially all that are currently in use for public elections (which simplify to IRV when there is only one winner).<br />
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All [[plurality voting system]]s are monotonic if the ballots are treated as rankings where using ''more than two ranks is forbidden''. In this setting [[first past the post]] and [[approval voting]] as well as the multiple-winner systems [[single non-transferable vote]], [[plurality-at-large voting]] (multiple non-transferable vote, bloc voting) and [[cumulative voting]] are monotonic. [[Party-list proportional representation]] using [[D'Hondt method|D'Hondt]], [[Sainte-Laguë method|Sainte-Laguë]] or the [[largest remainder method]] is monotonic in the same sense.<br />
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In elections via the single-winner methods [[range voting]] and [[majority judgment]] nobody can help a candidate by reducing or removing support for them, but as they are not ''ranked'' voting systems, they are out of the monotonicity criterion's scope.<br />
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==Instant-runoff voting and the two-round system are not monotonic==<br />
<br />
Using an example that applies to [[instant-runoff voting]] (IRV) and to the [[two-round system]], it is shown that these voting systems violate the mono-raise criterion.<br />
Suppose a [[president]] were being elected among three candidates, a left, a right, and a center candidate, and 100 votes cast. The number of votes for an absolute majority is therefore 51.<br />
<br />
Suppose the votes are cast as follows:<br />
<br />
:{| class="wikitable" border="1"<br />
|-<br />
! colspan=2 | Preference<br />
! rowspan=2 | Voters<br />
|-<br />
! 1st<br />
! 2nd<br />
|-<br />
| Right<br />
| Center<br />
| 28<br />
|-<br />
| Right<br />
| Left<br />
| 5<br />
|-<br />
| Left<br />
| Center<br />
| 30<br />
|-<br />
| Left<br />
| Right<br />
| 5<br />
|-<br />
| Center<br />
| Left<br />
| 16<br />
|-<br />
| Center<br />
| Right<br />
| 16<br />
|}<br />
<br />
According to the 1st preferences, Left finishes first with 35 votes, Right gets 33 votes, and Center 32 votes, thus all candidates lack an absolute majority of first preferences.<br />
In an actual runoff between the top two candidates, Left would win against Right with 30+5+16=51 votes. The same happens (in this example) under IRV, Center gets eliminated, and Left wins against Right with 51 to 49 votes.<br />
<br />
But if at least two of the five voters who ranked Right first, and Left second, would raise Left, and vote 1st Left, 2nd Right; then Left would be defeated by these votes in favor of Center.<br />
Let's assume that two voters change their preferences in that way, which changes two rows of the table:<br />
<br />
:{| class="wikitable" border="1"<br />
|-<br />
! colspan=2 | Preference<br />
! rowspan=2 | Voters<br />
|-<br />
! 1st<br />
! 2nd<br />
|-<br />
| Right<br />
| Left<br />
| 3<br />
|-<br />
| Left<br />
| Right<br />
| 7<br />
|}<br />
<br />
Now Left gets 37 first preferences, Right only 31 first preferences, and Center still 32 first preferences, and there is again no candidate with an absolute majority of first preferences.<br />
But now Right gets eliminated, and Center remains in round 2 of IRV (or the actual runoff in the Two-round system). And Center beats its opponent Left with a remarkable majority of 60 to 40 votes.<br />
<br />
===Estimated likelihood of IRV lacking monotonicity===<br />
<br />
Crispin Allard argued, based on a mathematical model that the probability of monotonicity failure actually changing the result of an election for any given [[European Parliament constituency|constituency]] would be 1 in 4000;<ref>[http://www.mcdougall.org.uk/VM/ISSUE5/P1.HTM Estimating the Probability of Monotonicity Failure in a UK General Election]</ref> however, Lepelley ''et al.''<ref>Dominique Lepelley, Frederic Chantreuil, Sven Berg: [http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6V88-3VW8PC1-1-3&_cdi=5864&_user=152381&_pii=0165489695008047&_origin=gateway&_coverDate=06%2F30%2F1996&_sk=999689996&view=c&wchp=dGLbVtb-zSkWA&md5=a8076aa10f6b2cd007416ce27d87a81d&ie=/sdarticle.pdf The likelihood of monotonicity paradoxes in run-off elections], Mathematical Social Sciences 31 (1996) 133-146</ref> found a probability of {{nowrap|397/6912 {{=}} 5.74%}} for 3-candidate elections.<br />
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Another probability model, the "impartial culture", yields about 15% probability. In elections with more than 3 candidates, these probabilities tend to increase eventually toward 100% (in some models this limit has been proven, in others it is only conjectured). Estimates of 5–15% order are easily confirmed in any probability model with "Monte Carlo experiments" and the aid of the "was it monotonic?" tests stated in the Lepelley paper.{{Citation needed|date=June 2011}} Nicholas Miller also disputed Allard's conclusion and provided a different mathematical model.<ref>[http://web.archive.org/web/20050523161035/http://research.umbc.edu/~nmiller/RESEARCH/DRAFT.POST.pdf Monotonicity Failure Under STV and Related Voting Systems]</ref><br />
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==Real-life monotonicity violations==<br />
<br />
If the ballots of a real election are released, it is fairly easy to prove if<br />
* election of a candidate could have been circumvented by raising them on some of the ballots, or<br />
* election of an otherwise unelected candidate by lowering them on some of the ballots<br />
<br />
would have been possible (nothing else is altered on any ballot). Both events can be considered as real-life monotonicity violations.<br />
<br />
However, the ballots (or information allowing them to be reconstructed) are rarely released for ranked voting elections, which means there are few recorded monotonicity violations for real elections.<br />
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===2009 Burlington, Vermont mayoral election===<br />
<br />
A real-life monotonicity violation was detected in the [[Burlington, Vermont mayoral election, 2009|2009 Burlington, Vermont mayor election]] under instant-runoff voting (IRV), where the necessary information is available. In this election, the winner Bob Kiss could have been defeated by raising him on some of the ballots. For example, if all voters who ranked Kurt Wright over Bob Kiss over Andy Montroll, would have ranked Kiss over Wright over Montroll, and additionally some people who ranked Wright but not Kiss or Montroll, would have ranked Kiss over Wright, then these votes in favor of Kiss would have defeated him.<ref>[http://www.rangevoting.org/Burlington.html Burlington Vermont 2009 IRV mayor election]</ref> The winner in this scenario would have been Andy Montroll, who was also the [[Condorcet winner]] according to the original ballots, i.e. for any other running candidate, a majority ranked Montroll above the competitor.<br />
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===Australian elections and by-elections===<br />
{{Refimprove section|date=April 2011}}<br />
Since every or almost every IRV election in Australia<br />
has been conducted in the black (i.e. not releasing enough information to reconstruct the ballots), nonmonotonicity is difficult to detect in Australia, even though thanks to the Lepelley ''et al'' probability estimates it seems safe to say that it must have occurred in over 100 of their elections. (The policy of Australia's election authorities not to release this data<br />
is justifiable on privacy grounds.{{According to whom|date=April 2011}} If rank-order ballots in an election with, say, 13 candidates, were released, even in a highly "anonymized" form, that would still provide enough information for a coercer to use to verify or deny that some voter had cast a pre-specified vote-pattern he'd demanded.)<br />
<br />
However, for the [[Australian federal election, 2010]], one article was aware of the non-monotonicity possibility: [http://andrewnorton.info/2010/08/16/why-labor-voters-in-melbourne-need-to-vote-liberal/ Why Labor Voters In Melbourne Need To Vote Liberal]. In 2009, the theoretical disadvantage of non-monotonicity worked out in practice in a state [[by-election]] in the [[South Australia]]n seat of [[Electoral district of Frome|Frome]]. The eventual winner, an Independent who was a town mayor, scored only third on the primaries with about 21% of the vote. But since the [[National Party of Australia]] scored 4th place, their preferences were distributed beforehand, allowing the Independent to overtake the [[Australian Labor Party]] Candidate by 31 votes. Thus Labor was pushed into third place, and their preference distribution favoured the Independent, who overtook the leading [[Australian Liberal Party]] candidate to win the election. However, had anywhere between 31 and 321 of the voters who preferred Liberal over Labor and Independent switched their support from Liberal to Labor, it would have allowed the Liberal to win the IRV election. This is classic monotonicity violation: the 321 who voted for the Liberals took part in hurting their own candidate.<ref>http://blogs.abc.net.au/antonygreen/2011/05/an-example-of-non-monotonicity-and-opportunites-for-tactical-voting-at-an-australian-election.html</ref><br />
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== See also ==<br />
*[[Voting system]]<br />
*[[Voting system criterion]]<br />
*[[Monotone preferences]] in consumer theory.<br />
*[[Monotonicity (mechanism design)]]<br />
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== References ==<br />
{{reflist}}<br />
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[[Category:Voting system criteria]]</div>Homunq