Talk:Independence of irrelevant alternatives

message: Sorry if this just me being dumb, but does "if one candidate (X) wins the election, and a new alternative (Y) is added, only X or Y will win the election." make sense?  Shouldn't it be "if one candidate (X) is running for election,..."?  Or do I just not get it?
—The preceding unsigned comment was added by 72.138.193.74 (talk) 00:52, 26 April 2007 (UTC)
replies: text: You just don't get it. (No offense intended -- it's a tricky concept.)  In the initial circumstance, X isn't the only candidate running; he's the winner.  The candidates might be A, B, C, and X.  X is currently winning.  The ieda of IIA is that if Y enters, that shouldn't change the winner to C.  If the choice among A, B, C, and X picked X as the best, adding Y shouldn't change that -- either X or Y should win the "new" election.  This seems intuitive, if you think that an election should produce a firm "ranking" of the candidates.  Except it turns out (as illustrated in the text) that while a single voter may have firm rankings in mind, society can collectively have cyclic rankings, saying "A is better than B, B is better than C, C is better than A." Auros 20:46, 2 May 2007 (UTC)
thread_title: Makes sense?

message: Is this finally a demand were Approval Voting works better than any Condorcet method (besides simplicity)?
: Yeah, Approval does satisfy IIA.  Of course, it doesn't satisfy the Condorcet criterion -- Arrow showed that no method can satisfy both the Condorcet criterion and IIA.  This is easy enough to see.  If you have a race that includes a cycle of A > B > C > A, and your method currently selects A, then if B drops out, you'll now have C > A, and C will win.  So the race is between A and C, under the control of B (an "irrelevant alternative" -- he is not the winner in either case).  This is why many analysts consider the IIA criterion flawed, and instead say that meeting Condorcet is good, and meeting the local IIA criterion is good, but total IIA is not important as long as those other two are met.  I'm not clear why another editor considered the example of how the existence of cycles trivially implies IIA failure a "special pleading" for Condorcet.  It seems, to me, critical to understanding Arrow's theorem and IIA's role in it. Rmharman 17:45, 14 June 2006 (UTC)
replies: text: Local IIA is clearly special pleading since it is designed to say "Condorcet methods satisfy IIA (except when they do not in which case it doesn't really matter)".  The role I see of IIA in Arrow's theorem is to show that democracy is complicated, and is vulnerable to tactical behaviour, and that there is no objectively optimal solution to the questions which result.  I see nothing special about the Condorcet criterion compared with many other criteria, and if you do then that is your point of view. --Henrygb 22:39, 14 June 2006 (UTC)
replies: text:  The point of LIIA is that there are some sets of preferences (cyclic ones) under which no method can satisfy IIA.  Satisfying IIA under all possible scenarios in which it is possible to satisfy it is not meaningless.  Saying that it's "special pleading" is like saying that building a "divide by 0" exception into a programming language is special pleading -- "your math program can solve all divisions, except the ones it can't, which don't really matter."  Well, yes, exactly.  We don't need to provide an "answer" to a divide by 0 request, other than, "that's meaningless."  In the context of cyclic preferences, IIA is meaningless.  Saying that "democracy is complicated" and all systems are vulnerable to tactical manipulation under some circumstances, doesn't mean it's impossible to make meaningful comparisons about how vulnerable a given electoral system is -- under what conditions odd behaviors might emerge, and how likely those conditions are in the real world.  As for whether Condorcet is special -- that depends on whether you think that majoritarianism is, in general, a good thing; Condorcet/Smith is the clearest majoritarian criterion, though note that even the most basic version, "a candidate with a majority of first choice votes must win," is contradictory to IIA, as I showed in the example in the article.  The only way to satisfy IIA is to have an "election" that doesn't actually consider the will of the voters -- either have it be dictatorial, or random, or something silly like that.  In any case, I don't privilege Condorcet above all other concerns -- I'm fine with the variety of Condorcet failure that can occur under Approval Voting, for example, as long as voters have access to good polling information and can make choices with some understanding of the consequences -- but in general, a Condorcet-winner seems to have a pretty strong argument that they're the majority choice. Rmharman 21:13, 23 June 2006 (UTC)
replies: text: There are plenty of methods that satisfy IIA.  Not only are there  dictatorial methods, random methods, and others, but the cardinal methods (such as Approval voting and Range voting) have a particular attractiveness.  But they have other issues.    Condorcet methods and the criterion tend to presume one-dimensional politics and often lead to the election of inoffensive candidates rather than contraversial ones.  You may like that, but it is still a POV.  --Henrygb 01:37, 24 June 2006 (UTC)
replies: text: Approval and Range do not meaningfully "satisfy" IIA, they simply fail to provide the information that would allow you to determine that it has been violated.  Given a description of voters' ordinal preferences, one can make guesses at how they will vote in Approval or Range, but the two systems don't map transparently to each other.  If you define the right sort of strategy for mapping ordinal preferences to cardinal ratings, you can recreate the example, where A wins with B in, but C wins with B out.  Dictatorial or random methods select a winner, but it's hard to justify calling either an election method.  As for your last line, it seems to be generally understood that Condorcet favors centrism somewhat less than Approval or Range, which, at least in theory, favor a "broad" strategy (aiming to provide middling utility for many people) over a "deep" one (providing high utility for few people).  Condorcet doesn't clearly favor either variety of strategy.  The notion that centrism is automatically equivalent to being "inoffensive" is simplistic, and probably wrong in many cases -- if one imagines a re-run of the 2000 US presidential election using AV amongst Bush, Gore, and McCain, it seems highly likely that McCain would win, precisely because he was (at the time) seen as being more bold than either candidate.  "Centrist champions" tend to get driven out by first-past-the-post and other Duvergerian methods. Rmharman 22:16, 22 August 2006 (UTC)
replies: text: What a strange way of looking at the world.  The key point about range voting (for honest voters) is that it not only allows voters to say whether they prefer one candidate over another, but also allows them to say by how much they care.  So it allows more information (assuming the range is big enough) than ordinal preference voting.  There is no failure to provide information. --Henrygb 23:27, 22 August 2006 (UTC)
replies: text:  I'm sure you can see why Approval doesn't provide full ranking info, but my apologies, I should've been clearer on why I don't consider Range to provide the full ranking info either.  If we can safely assume honesty ratings, it would provide rank info.  But there is a very strong incentive for voters whose real utilities (on a scale from 0 to 99) are something like "Minor Party = 99, Major Party A = 51, Major Party B = 0" to make an exaggerated claim of utility for Major Party A.  They do not expect Minor Party to win, and if they want to maximize their expected utility from the election, they need to maximize the difference in their ballot between the two likely winners. Thus, they will actually cast an Approval-style ballot, with Major Party A given the same maximum score as Minor Party.  You follow?  In any case, I have, in recent months, started to come around to the view that rating-based systems may, nonetheless, have advantages over ranking-based systems, even if the ranking system collects approval info.  The Center for Range Voting has some interesting work from KVenzke suggesting that even Condorcet does not fully escape "duopoly problems".  I haven't decided yet whether I think their cases where betraying a favored third-party in favor of a major party is encouraged are things that a strategic voter could realistically anticipate. Rmharman 17:53, 22 November 2006 (UTC)
replies: text: I accept the point that for some people there is an incentive for tactical voting (I wrote the final paragraph of Range_voting#Example), but even then most of them might give Major Party A 98 rather than 99.   And if they saw preventing Major Candidate B from winning as their overwhelming priority then it suggests they see no significant difference between Minor Party and Major Party A. --Henrygb 22:57, 22 November 2006 (UTC)

text: I agree that Approval and Range cannot be said to satisfy IIA. With approval, it cannot be said to satisfy or violate anything, because there is no presumed, consensus strategy with Approval. The leading Approval Voting organization doesn't even have an endorsed strategy. If a voter simply approves of his or her top choice, then it is exactly plurality and doesn't satisfy IIA. If a voter approves of a front-runner and everyone s/he likes better than the front-runner, then it doesn't satisfy IIA either. So what exactly is the strategy by which Approval is evaluated? With Range Voting, the advocated strategy, so I've heard, is to give your favorite candidate the top score, your least favorite the bottom score, and to score everyone else relatively in between. This can clearly suffer from the same "center squeeze" scenario that IRV does, which doesn't satisfy IIA either. Progressnerd 22:02, 12 June 2007 (UTC)
replies: text: I believe the optimal strategy for Approval is "work out your expected utility from the election, and approve all candidates who give you greater than expected utility". This will nearly always produce a set of approved candidates which includes at least one, but not all, of the front-runners. If a *non-front-runner* drops out, your expected utility will hardly change - the non-front-runner is pretty much by definition altering that weighted average very little. If a *front-runner* drops out, however, this may dramatically alter your expected utility, and lead to a changed set of approved candidates.
Jack Rudd (talk) 20:58, 22 May 2008 (UTC)
thread_title: Approval and range voting

message: LIIA implies Condorcet (and Smith), but Condorcet (and even Smith) doesn't imply LIIA. KVenzke 03:21, 20 June 2006 (UTC)
replies: text: Is there a name for the subset of Condorcet methods that satisfies LIIA? Rmharman 21:13, 23 June 2006 (UTC)
replies: text: I don't know of a name for this subset of methods. I would have to just call them "methods which satisfy LIIA." KVenzke 19:41, 25 June 2006 (UTC)

text: The point is that IIA is incompatible with some very basic ideas about democracy and social choice (like "if there are only two options, the one preferred by a majority should win") and as such is a very silly criterion to be applying to election methods.  Thus, the only way the concept of "irrelevant alternatives" can have any meaning at all is under LIIA, where candidates that are members of a Smith set of more than one element are not mis-defined into the "irrelevant" class.  IIA is badly named! Rmharman 22:16, 22 August 2006 (UTC)
replies: text: It is fine to say that the majority criterion and independence of irrelevant alternatives conflict.  That does not make either good, bad or silly. But you are displaying your POV when you say "the only way the concept of irrelevant alternatives can have any meaning at all is under LIIA" - it simply is not true. --Henrygb 23:27, 22 August 2006 (UTC)
thread_title: LIIA

message: I deleted the piggybacking addition on my own parenthetical comment on the Morgenbesser anecdote because if it is correct (and I'm not convinced it is -- it needs at the least more exposition) it should be a separate "technical" paragraph on its own, not overloading what is meant to be an illustrative non-technical comment on an illustrative anecdote.   The meaning of the addition is also not clear to me -- that the analogy is imperfect because the preference rankings could shift non-transitively with a new option even *after* the choices have been frozen?  This seems meaningless to me -- how can one take account of the new option at all if the choices are already frozen?  142.103.168.33 04:19, 12 July 2006 (UTC)
replies: text: I think that the comment was useful, although you're probably right in that it didn't belong in your parenthetical.  The comment seemed to discuss the fact that preferences are taken as given for voting systems---anything that acts to change that is, strictly speaking, outside of the scope of the field.  This is why there is a difference between runoff elections and IRV: preferences can only be stated once, frozen, for IRV, while they can change for various reasons in a true runoff election. See, for example, Corks' "Evaluating Voting Methods" (p. 3) CRGreathouse 03:12, 14 July 2006 (UTC)
replies: text: I deleted the addition "not exactly rational" added by Noe to my own parenthetical comment (originally entered from another IP address) on the Morgenbesser anecdote because it misses the whole point of my comment -- that the apparently irrational non-transitive nature of Morgenbesser's choice could be a perfectly sensible and rational consequence of the "irrelevant" alternative changing the internal state of the decision-maker.  There is nothing "irrational" in human cognition in something reminding you of something else -- the "else" thing then being used as the basis for a decision where the original reminder plays no part. 137.82.82.145 20:03, 22 November 2006 (UTC)
thread_title: Preferences set

message: I don't understand why LIIAC is considered a weaker version of IIAC.  IIAC doesn't imply LIIAC, does it?  We should mention this if that's the case, or include a short proof if it isn't.  Also, does someone have a reference to the Young & Levenglick paper introducing LIIAC? CRGreathouse  22:33, 13 July 2006 (UTC)
replies: text: You are correct.  It is easy enough to produce a range voting example (so meeting IIA) where the new candidate wins despite not being in the Smith set. --Henrygb 09:45, 14 July 2006 (UTC)
replies: text: Are there any methods that adhere to both IIAC and LIIAC? Range voting has IIAC but not LIIAC; the Schulze method has LIIAC but not IIAC; the Borda count has neither IIAC nor LIIAC.  I'm just trying to see if there's one with both -- they seems mutually exclusive, even though related. CRGreathouse 17:36, 16 July 2006 (UTC)
replies: text: Silly me -- LIIAC → Smith criterion → majority criterion → not IIAC. CRGreathouse 16:06, 19 July 2006 (UTC)
thread_title: LIIAC

message: A recent edit deleted references in the article to Arrow (1951), b/c his usage of 'IIA' is different from the usage of 'IIA' in the rest of the article.  One way of handling the ambiguity is with a Wikipedia:Disambiguation page and separate articles for each page.  Another way is to define and discuss each uaage within the article itself (as was done in the article Qualitative economics).  (This has already been done with the section IIA in econometrics.)  Then the fact of different usages should be noted in the lead section. 
Arrow's usage of the same term is spelled out in the link to Arrow in the article.  There's also a nice intuitive statement in Kenneth Arrow.    
The current intro to the article is 
:Independence of irrelevant alternatives (IIA) is an axiom often adopted by social scientists as a basic condition of rationality. The axiom states: If A is preferred to B out of the choice set {A,B}, then introducing a third, irrelevant, alternative X (thus expanding the choice set to {A,B,X} ) should not make B preferred to A. In other words, whether A or B is better should not be changed by the availability of X. 
In Arrow's usage, the second sentence could be rephrased as:
:IIA states: If A is preferred to B out of the choice set {A,B}, then no matter what preferences are for an unavailable third alternative X, B should not selected by the voting rule over A. 
Example (adapted from Sen, Collective Choice and Social Welfare, 1970, p. 37): the choice between Hillary and McCain, given preferences of voters for each relative to the other in 2008, is not to be affected by their preferences for say George Allen (who by hypothesis is not on the ballot) in 2008.  Thomasmeeks 15:59, 2 December 2006 (UTC)
replies: text: I don't think they're different.  There are arguments about minor differences in IIA (see the classic 1973 article of Paramesh Ray in Econometrica), but the concept of IIA in social science is literally based on that of Arrow. If there is a difference between the two uses you list, it's entirely because the one you list for Arrow is phrased wrongly. That's not the ordinary way of phrasing it. I don't have his seminal 1951 book (I've read it, though), but in the original 1950 paper he talks about removing an alternative, which can be clearly seen as equivalent to adding one. CRGreathouse (t | c) 21:08, 2 December 2006 (UTC)
replies: text: I'm assuming that the wording in the lead correctly states IIA.  If so, that wording requires changing to produce Arrow's IIA.  The Kenneth Arrow cite above and my example above are dissimilar to those in the IIA article.  The Econometrica cite in the IIA article (which I have not read) may bring out the same difference that I am asserting. Thomasmeeks 02:24, 3 December 2006 (UTC)
replies: text: To be more explicit, in the article addition of X to those that are available could flip the observed choice of A or B (the spoiler effect of X not being chosen but affecting the choice of A or B), but in Arrow, availability of X removes tha applicability of his IIA condition to the choice of A & B.  In his usage, what makes a variable 'irrelevant' is not that it is not chosen but that it is unavailable.  Thomasmeeks 12:29, 3 December 2006 (UTC)

text: P.S. Concerning the last sentence, a couple more results from the Ray (1973) article supporting it:  Ray proves that the IIA(R-M)  condition (congruent with the current IIA-article usage) is consistent with Arrow's other conditions. That is, Arrow's impossibility theorem cannot be derived  with IIA(R-M) instead of IIA(A) (p. 989, Theorem 2).  Second, as between IIA(R-M) & IIA(A), neither can be derived from the other.  Still, there is overlap in the conditions, and certain kinds of information restriction can reduce IIA(R-M) to IIA(A) in which case the impossibility result recurs, though in that case IIA(R-M) adds nothing to the information restriction (pp. 990-91).  Thomasmeeks 02:09, 7 December 2006 (UTC)

text: Works for me (if any background info deemed necessary to make that exposition self-contained and clear to those who had not read the source article were worked in).  Thomasmeeks 11:43, 3 December 2006 (UTC)
replies: text: I'll work on this.  As the above suggests, there are different ways IIA could go, from new sections to new articles to spinning off sections to a disambiguation page.  In my edits, I'll try to respect the integrity of the whole article and make edits as plain as I can.  Improvements are always welcome as is rational criticism.   (minor edit) Thomasmeeks 11:16, 7 December 2006 (UTC)
thread_title: Possible new section in article:  IIA in Arrow's social choice theory

message: Maybe it's just me, but it seems like the example is flawed. 
"Note that the social choice ranking of [A, B] is dependent on preferences over the irrelevant alternatives [B, C]."
How can B be considered irrelevant when it is the winning candidate when it is switched? It's just like saying: There are three parties, 5 voted A, 3 voted B, 3 voted C. Because B and C lost, giving the voted from C to B shouldn't matter. (which is illogical)
I'm trying to think of a better example here..203.116.243.1 (talk) 03:17, 27 September 2010 (UTC)
replies: text: Just a quick calculation: a,b,c with results spoiled by c: (cab = 2)(abc = 1)(bca = 3)(cab = 2)(bca = 1) I'm not sure if this is the most efficient spoiled situation. Basically, without c, a beats b. With c, a gets eliminated by runoff, then b beats c.203.116.243.1 (talk) 03:46, 27 September 2010 (UTC)
thread_title: Instant-runoff voting: Flawed Example?

message: I don't think the example adduced here is a clear example of IIA; I'm not sure I even think it's a flaw.  One intent of Borda voting is to give "partial credit", as it were, for preferred also-rans.  If enough credit is added to an also-ran who's already close to winning, it can push them over the top.
I think a better example is as follows.  Let's suppose that five voters rank alternatives {A, B}; three prefer A to B and two prefer B to A.  If we score two points for a first choice and one point for a second choice, A wins over B, 8 points to 7.
Suppose a third alternative C is introduced.  Suppose also that the three voters who preferred A find C the worst of the three (A, B, C), but the two voters who preferred B put C in second place (B, C, A).  If we score three points for a first choice, two points for a second choice, and one point for a third choice, we now find that B wins with 12 points, A is second with 11 points, and C brings up the rear with 7 points.  In short, introducing a third option that did not affect the individual relative rankings of A and B nevertheless influenced the overall result of the vote. BrianTung (talk) 18:58, 1 February 2011 (UTC)
thread_title:  Borda example flawed? 

message: The definition of Local Independence of Irrelevant Alternatives (LIIA) in the article does not at all match the definition provided by Peyton Young in his 1993 book Equity in Theory and Practice.
Here are two equivalent ways to express the real LIIA: 
(1) If the alternative that won or the alternative that finished last is deleted from all the votes, then the relative order of finish of the remaining alternatives must not change. 
(2) If a subset S of the alternatives are together in the order of finish--in other words, no alternative outside S finishes between or tied with any alternatives in S--then the relative order of finish of the alternatives in S must not change if all alternatives outside S are deleted from the votes.
Young's LIIA is much stronger than the criterion listed in the article, and it's failed by one of the two voting methods that the article mentions as satisfying LIIA, Schulze's method. (Ranked Pairs satisfies it, as does Peyton Young's method. Young's method also satisfies a weak reinforcement criterion--if two collections of votes separately produce the identical order of finish, then the combined votes must produce that same order of finish. Young's method fails independence of clone alternatives, however, which is much more important than reinforcement since it is fairly easy for a minority to nominate slightly inferior clones but hard or impossible for a minority to partition the voters.)
Depending on the comments, I intend to correct the article. SEppley (talk) 17:08, 27 February 2012 (UTC)
replies: text: A couple of weeks elapsed with no objection, so I corrected the article. SEppley (talk) 17:51, 13 March 2012 (UTC)
thread_title:  Incorrect definition of LIIA! 

message: Here are the problems addressed in the edit:
The former paragraph reads:
"
IIA is too strong to be satisfied by any voting method that reduces to majority rule when there are only two alternatives. Most ranked ballot rules do so, while Approval and Range can pass IIA because they do not necessarily reduce to majority rule.[5] Any comprehensive use of strategy that makes Approval or Range reduce to majority rule when there are only two candidates will make those methods fail IIA as well. (Even if only one voter is an optimizing voter, it is possible to construct a tied or nearly-tied example to show IIA can be violated.)
"
Problem #1: To comply with IIA, a method must pass every case. Any failure case results in non-compliance. Therefore, there is a problem with the rhetorical structure: "Approval and Range can pass IIA because they don't necessarily reduce to majority rule". This does not logically follow they pass IIA because all that is needed is one case of reducing to majority rule that causes failure.
Problem #2: “Comprehensive use of strategy” by voters is not required to make Approval and Range fail IIA. Nor is it required that any voter is an “optimizing voter”. It only requires that a voter might be minimally rational – i.e. that it is at least possible they do not vote against their interest – i.e. abstain from voting. As long as we do not assume that certain voters will necessarily abstain – i.e. we do not necessarily exclude them from representation,  then all rating methods fail IIA. Specifically we are just allowing even the possibility that a voter having meaningful preferences for A>B, might cast a positive vote for A in a two candidate election (rather than abstain), and thus allowing Approval and Range to fail IIA. This failure is trivially shown just as Plurality voting fails to resolve common vote splits. Making clear the minimum requirement for failure of IIA is editorially vital to the thesis of this section, that IIA is too strong.
PS last two paragraphs of this section may require editing or removal, or moving to Majority Criterion
Filingpro (talk) 15:45, 5 May 2015 (UTC)
thread_title:  Edits To Criticism of IIA 

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thread_title:  External links modified 

message: This section contains the wonderful sentence:
"Arrow's IIA does not imply an IIA similar to those different from this at the top of this article nor conversely.[7]"
...
What?
"does not imply": ok, se we're a no
"similar to": um, like
"those different from": wait, unlike
"this at the top": ok, I think this is actually "those at the top" or referring to the other IIA definition above
"nor conversely": .. wait, the opposite?  Is this logical converse?  Wait, let me rewrite:
not like those unlike to the other defintion, or the other way
not unlike the other definition, and not like?
like the other def, and also not?
I really feel this is saying that "Arrow's IIA is different from the earlier definition, and it doesn't imply the earlier definition.  Also the earlier definition does not imply Arrow's definition of IIA. But they have similar intuitions."  But this is I think reading a lot into it and prognosticating meaning where I feel it is very confused.  I actually don't know which implication directions hold, if any.  I came here to find out.  — Preceding unsigned comment added by Ex0du5 5utu7e (talk • contribs) 06:04, 20 August 2020 (UTC)
thread_title:  Confusing wording in section "In social choice (under Voting Theory)"