[EM] Approval vs. IRV (hopefully tidier re-send)

[Ted Stern]

On 28 Nov 2011 20:24:37 -0800, Chris Benham wrote:
>
> Matt Welland wrote (26 Nov 2011):
>
>     Also, do folks generally see approval as better than or worse than IRV?
>    
>     To me Approval seems to solve the spoiler problem without introducing
>     any unstable weirdness and it is much simpler and cheaper to do than
>     IRV.
>
> If we are talking about the classic version of IRV known as the
> "Alternative Vote" in the UK and "Optional Preferential Voting" in
> Australia, then I see IRV on balance as being better than Approval.
>
> The version of IRV I'm referring to:
>
> *Voters strictly rank from the top however many or few candidates
> they wish.  Until one candidate remains, one-at-a time eliminate
> eliminate the candidate that (among remaining candidates) is
> highest-ranked on the fewest ballots.*
>
> The "unstable weirdness" of Approval is in the strategy games among
> the rival factions of voters, rather than anything visible in the
> method's algorithm.
>
> Approval is more vulnerable to disinformation campaigns. Suppose
> that those with plenty of money and control of the mass media know
> from their polling that the likely outcome of an upcoming election
> is A 52%, B 48% and they much prefer B.
>
> In Approval they can sponsor and promote a third candidate C, one
> that the A supporters find much worse than B, and then publish false
> polls that give C some real chance of winning. If they can
> frighten/bluff some of A's supporters into approving B (as well as
> A) their strategy can succeed.
>
> 47: A
> 05: AB (sincere is A>B)
> 41: B
> 07: BC
>
> Approvals: B53,   A52,  C7

I find this example contrived.

 * If mass polling is available, many people will be aware of the
   52/48 split between A and B ahead of time.

 * Corruption is a separate issue.  With proper election funding
   control, support for C would be restricted.

> Approval is certainly the "bang for buck" champion, and voters never
> have any incentive to vote their sincere favourites below
> equal-top. But to me the ballots are insufficiently expressive by
> comparison with the strict ranking ballots used by IRV.

I agree.

Approval-Bucklin (AKA ER-Bucklin) has the advantage in your contrived
example of allowing the A > B voters to add B at a lower rank, which
would not count unless neither A nor B achieves a majority.

In many cases, it would not be necessary to rate candidates at the
second (or lower) choice option, but having that option increases the
available nuance of the vote.

> IRV has some Compromise incentive, but it is vastly less than in
> FPP.  Supposing we assume that there are 3 candidates and that you
> the voter want (maybe for some emotional or long-term reason) to
> vote your sincere favourite F top even if you think (or "know") that
> F can't win provided you don't thereby pay too high a strategic
> penalty, i.e. that the chance is small that by doing that you will
> lose some (from your perspective positive) effect you might
> otherwise have had on the result.

However IRV does impose a false choice -- that you must rank your
preferences separately, no equal ranks allowed.

> In FPP, to be persuaded to Compromise (i.e.vote for your compromise
> "might win" candidate C instead of your sincere favourite F) you
> only have to be convinced that F won't be one of the top two
> first-preference place getters.
>
> In IRV if you are convinced of that you have no compelling reason to
> compromise because you can expect F to be eliminated and your vote
> transferred to C. No, to have a good reason to compromise you must
> be convinced that F *will* be one of the top 2 (thanks to your vote)
> displacing C, but will nonetheless lose when C would have won if
> you'd top-voted C.
>
> In my opinion IRV is one of the reasonable algorithms to use with
> ranked ballots, and the best for those who prefer things like
> Later-no-Harm and Invulnerability to Burial to either the Condorcet
> or FBC criteria.

But are these the criteria we really want to achieve in a
single-winner election?  To say that LNH is the most important
criterion is, at its most basic level, an emotional argument.  While
effective in persuading the electorate, I think what we really want to
look for is a method that does a good job of finding the candidate
closest to the center of the electorate, while resisting strategic
manipulation.

Ted
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