I agree with what /u/zestyping says in the other comment, so I won't repeat it. But here's a video explaining the strategic voting issue: https://www.youtube.com/watch?v=JtKAScORevQ
I still find the nonmonotonicity of instant runoffs problematic. Something seems wrong about a system where you can lose by gaining votes. I'd expect instability to be higher than a similar system that is monotonic.
The non-monotonicity tactical voting possibility is overblown: it only really works in the toy "which is the favourite pizza of these 30 students" examples.
In real world elections, the conditions where it is even theoretically possible arise only rarely (A > B > C with three choices, but B > A and A > C with two choices, and A's lead over B significantly greater than B's lead over C in the three choice scenario, and A's lead over C in the two choice scenario more than twice B's lead over C in the three choice scenario) and more importantly, they're not predictable enough beforehand. Advocating this kind of tactical vote stands at least as much chance of hurting your candidate as helping them, so nobody does it.
When you analyse real-world large scale IRV elections, you find that cases where the IRV winner isn't the Condorcet winner are rare, and this balances against the very real benefit of having a counting method that is easy to explain and understand.
But expressing your preferences doesn't actually accomplish anything useful.
You can do it, but what does it get you?
Instant runoff is not monotonic: there are times when the best strategy is to vote your favored candidate second and your second choice first so that the second choice can push out your least favored candidate.
Monotonicity intuitively seems like it would be desirable, but I'm struggling to see how the effect described in the video actually matters in practice. It seems vanishingly unlikely that candidates 2&3 would be eliminated in a tie (and you can just change the rules to force a re-vote/runoff in that case if you really think it's a bad outcome, similar to how you'd handle a tie in FPTP). And even then, the outcome of picking the first candidate when 2&3 are tied doesn't seem explicitly unfair. I'm sure there are other cases where this can show up too though, and maybe some of those do seem unfair.
I can see how ex post, one could say "if I had ranked my choice lower, they would have done better", but it's not clear to me that this property actually informs any real-world voting strategy ex ante.
Basically, this seems like a somewhat academic property that most voters probably wouldn't care about, so it's not clear that a lack of monotonicity would actually impede IRV from popular acceptance.
Read "22. Does RCV satisfy the monotonicity criterion?"
Say, there's 3 candidates. You want A to win. You think A will lose to B in a 1-on-1 match, but would win C. So you vote C-A-B (or C-B-A, doesn't matter) to increase the likeliness that B got eliminated in the 1st round. Of course, you're now saying that you prefer C over A in a 1-on-1 race. But that's fine because other people will vote for A.
Concretely, let's say this is people's actual choice.
31 B-A-C; 39 A-B-C; 30 C-B-A
B wins.
Now, if 2 people preferring A (voted A-B-C) strategically change their vote to C-B-A.
> You don’t have to game theory your choice of the lesser of two evils, you rank who you prefer
You "have" [1] to game-theory your choice no matter what the voting system is. Ranked-choice is apparently believed [2] to be less susceptible to tactical voting than some other systems, though. (Not sure if this is quantitative or qualitative.)
> IRV may incentivize forms of tactical voting (such as compromising) when voters have sufficient information about other voters' preferences, such as from accurate pre-election polling. FairVote mentions that monotonicity failure can lead to situations where "having more voters rank [a] candidate first, can cause [the candidate] to switch from being a winner to being a loser."
Yeah, I'm not sure where the qualms about monotonicity are coming from. In an IRV the lower-ranked preferences do not count until the higher-ranked preferences have already been eliminated for not reaching majority
Instant-runoff voting can punish voters for prioritizing a 3rd party candidate. IRV does not satisfy the monotonicity criterion. See this article [1] for more information.
In general, IRV is a poor system that is almost as bad as plurality voting in terms of how the outcomes reflect the voters' preferences (plurality voting is what the United States uses for most elections).
There are many systems that are better than IRV or plurality. I like approval voting [2], which is a system where you can vote for any number of candidates (instead of one candidate like plurality). Unlike IRV, you don't rank your choices. Approval voting is very simple (the simplest option other than plurality) and produces good outcomes.
These charts[0], which I did not create, should explain why instant runoff voting is less than ideal. In particular, its nonmonotonicity (a candidate getting more votes can cause them to lose) is problematic, IMO.
I highly recommend CES's video on voting methods that highlights the non-monotonicity of IRV [1]. I would prefer the adoption of simpler models that can be implemented and understood by voters easily, such as approval or score voting.
Since this is a subject I find fascinating, I'll add this link that I first encountered here on HN long ago: http://zesty.ca/voting/sim/
It's a mathematical model and accompanying explanation that shows that simple plurality (first past the post) voting produces bi-polarization, while other voting methods like approval voting do not. It also shows that the instant-runoff voting method that people are trying to replace FPTP with is non-monotonic, meaning that gaining a few points of support can actually hurt a candidate.
I think the more meta problem is that the people who agree that plurality is bad can't agree on what a better system should be :)
IRV actually encourages strategic voting, see e.g. http://mattbruenig.com/2014/11/15/instant-runoff-voting-and-.... For a concrete example, if in the USA we had an IRV election between Trump, Clinton, and Sanders in 2016, there are some (not unrealistic) scenarios where it's possible that your (honest) choice to vote 1. Sanders 2. Clinton 3. Trump would cause a Trump victory, whereas a strategic vote of 1. Clinton 2. Sanders 3. Trump would cause a Clinton victory.
You can argue that approval voting also can lead to strategic voting, but at least with approval voting you won't accidentally cause your least favorite candidate to win by voting honestly.
You should look at Condorcet methods, which as a group are conceptually similar to IRV but are more effective at capturing the intent of the electorate. For example, the above scenario can't happen with Condorcet methods. (Again the problem there is that there are actually many of them, and you have to agree on which specific one you want. But all are better than IRV).
IRV isn't the way forward, and it's unclear to me why organizations like FairVote don't endorse another method. (They have an article about it here but the reasoning doesn't really make sense to me. http://www.fairvote.org/why-the-condorcet-criterion-is-less-...)
This is completely false. Pretty much all voting systems are susceptible to tactical voting, e.g. gaming the vote. This was formally proven via the Gibbard–Satterthwaite theorem[1], which showed that the only way for a voting system to be ungameable was to either have a dictator (a certain special voter who always picks the winner), or to pre-constrain the output space to only two alternatives.
This critique certainly applies to Instant-Runoff Voting, the algorithm in question here. It is known that IRV often incentivizes voters to not rank their favorites first [2].
Instant runoff voting has the problem of nonmonotonicity. A simpler ranking system without rounds of elimination and vote shuffling would be better, IMO.
I've seen some voting systems include a "delegation" feature, where instead of submitting a full ranking of candidates (or whatever), you can just vote for a single candidate and your full ballot will be determined by their full ballot. What you're describing seems to be basically IRV but where you are forced to delegate?
(OK, I guess the runoff isn't truly instant, but still.)
I worry that this might have the same sort of monotonicity problems as IRV normally does, only in terms of the candidates' ballots instead of the voters'.
In particular, IRV satisfies the important "majority criteria," e.g. if the majority of people vote for a candidate, that candidate should win. Approval / range voting fail this criteria. See, again, Arrow's Theorem. http://archive.fairvote.org/monotonicity/.
Also, wikipedia has an example of a non-monotonic result here: https://en.wikipedia.org/wiki/Monotonicity_criterion#Instant...
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