[EM] Quantifying manipulability

[barnes99]

>===== Original Message From election-methods-list at eskimo.com =====
>From: "Narins, Josh" <josh.narins at lehman.com>
>To: election-methods-list <election-methods-list at eskimo.com>
>Subject: RE: [EM] Sports and 'The Condorcet Mindset'
>
>Well, with IRV, you could only manipulate the outcome if you had
>near-perfect information on the votes of other people, otherwise your as
>likely to undo your own strategy as fulfill it.
>
>Borda, by comparison, can be "utterly" manipulated with no information,
>since you can bury someone all the way down to the bottom rung with impact.
[...]

Josh:

Not true. By the usual definition of "manipulation," an insincere vote must be 
rewarded, not punished, to qualify as manipulation. The Borda Count will 
punish an insincere vote in some cases, and that is actually an incentive to 
vote sincerely. If too many people sincerely put Nader between Bush and Gore, 
and voted either Bush>Nader>Gore or Gore>Nader>Bush, Nader would have been 
elected. Therefore, you need some information to successfully manipulate the 
BC, and there is only a limited an envelope of manipulability. In some cases, 
depending on the sincere preferences, a given voter type (such as a 
Gore>Bush>Nader voter) cannot successfully manipulate the outcome to his own 
advantage at all.

As a start, maybe we could quantify the number of voters who could 
successfully manipulate the outcome in a particular example. Here's a simple 
example to kick off the game:

1 ABC
1 ACB
1 CAB
1 CBA
1 BCA
1 BAC


Is the BC is more manipulable than IRV or plurality vote, in this case? If so, 
precisely how much more (in numbers)?



SB

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