[EM] Approval vs. IRV

[C.Benham]

Juho Laatu wrote (29 Nov 2011):

> We may compare IRV also to the other commonly used single-winner 
> method TTR. To be brief, one could say that IRV is better than TTR 
> since it has more elimination rounds. IRV's problem in this comparison 
> is that it collects so much information that one can, after the 
> election, see what strategies would have paid off. In TTR one may have 
> very similar problems but people stay happier since they can not see 
> the problems. They can't see for example what would have happened if 
> some other pair of candidates would have made it to the second round. 
> Spoilers may exist but they remain undetected, or at least unverified.


Yes, IRV is much better than TTR partly for that reason. IRV simulates 
"everyone gets one vote each, eliminate one candidate, repeat until one 
remains"  (a process I think is called the Exhaustive Ballot)
except that voters can't sit out a round or two and then come back in, 
and they have to keep voting consistent with their ranking that they 
give at the beginning (so if in the first round they vote for X they
have to keep voting for X until X is eliminated or wins).

This last feature is a big positive because it makes using the devious 
Push-over strategy much more difficult and risky. In TTR if you are 
confidant that your favourite F will make the second round
without your vote (but not make the majority threshold even with your 
vote) you might be able to improve F's chance of winning by voting in 
the first round for a "turkey" T that you are sure that F
can pairwise beat with your vote.

In IRV if you try that and you succeed in causing the final (virtual) 
runoff to be between F and T, F has to win with you still voting for T.

> I'd like to add that IRV is an algorithm for those that want to favour 
> the large parties. 


The main thing that favours large parties is legislators elected in 
single-member districts versus some form of  PR in multi-member 
districts. But yes, IRV is a bit biased towards slightly off-centre
candidates whereas Approval has a strong bias toward centrist 
candidates.  In Approval it is just possible to have a surprise centrist 
winner, by getting all the approvals of voters in the centre
(with maybe some being exclusive approvals) and approvals from some of 
the wing voters who fear the opposing wing candidate more than they like 
(or are hopeful about) their own.


Chris Benham