[EM] Fixing IRV
Richard Moore
rmoore4 at home.com
Sat Aug 11 16:46:34 PDT 2001
Anthony Simmons wrote:
> The answer falls out of its own accord if I just
> reformulate the problem in sufficiently rigorous
> doubletalk.
>
> Let the number of voters be N, and the number of ways of
> marking a ballot (valid ways plus one general way called
> "invalid") be m. Then there are N^m distinct ways that all
> of the ballots can be marked. (If you like, you could
> include all registered voters in N, and include one more way
> of marking the ballot, "unvoted".)
>
> Each way of marking the ballots maps to an outcome, which is
> a ranking of the candidates, allowing for whatever
> peculiarities, such as ties, might be allowed.
>
> Call this function, which maps sets of marked ballots to
> outcomes, an "election method". Each election method, being
> a function, is just a set of ordered pairs; each pair
> contains one set of ballots and the outcome they map to.
>
> Each election method is a finite set (with N^m elements) of
> ordered pairs. Since every election method is a finite set,
> there is an effective, if somewhat tedious, method of
> comparing two of them to see if they are equivalent --
> compare their members. One of those "at least in principle"
> things. Since the number of election methods is also finite,
> it is, again at least in principle, possible to compare a
> given election method with all other election methods (or
> with a subset of them) to see if they are equivalent.
>
> Therefore, it is possible (yes, "at least in principle") to
> prove that a given election method is or is not equivalent to
> a non-elimination method. This does *not* mean there's a
> feasible method.
>
> Life is so easy with finite sets.
This certainly could be used "in principle" to prove things
about elections for N <= N1 and m <= m1, where N1 and m1 are
abritrary limits. But unless you can construct a
mathematical induction proof, this result doesn't apply for
N > N1 or m > m1, and you have to repeat the process with
these larger numbers. It then follows that the problem is
infinite in scope.
Example: The method that says "elect the Borda winner unless
there are more than 1 million voters, in which case elect
the plurality winner" is equivalent to Borda as long as the
number of voters is less than or equal to 1 million.
In this light, of course, the number of methods is also
infinite.
Richard
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