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

[Chris Benham]

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 

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. 

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. 

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. 

Chris Benham 
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