[EM] Consensus?: IRV vs. Primary w/Runoff
fsimmons at pcc.edu
Tue Jan 29 17:18:31 PST 2002
On Tue, 29 Jan 2002, Steve Barney wrote:
> Do we have a consensus that the instant runoff vote (IRV) is MATHEMATICALLY
> better than the common two step plurality vote (primary) with a follow-up
> runoff between the 2 top plurality vote getters? It seems to me that it cannot
> be worse, given that manipulation is bad. As far as I can see, the only
> mathematically provable difference between them is that IRV is less
> manipulatable. They share the same faults, including non-monotonicity. With 3
> candidates, I believe they are mathematically identical.
I'm not sure what you are trying to say here. You conclude by saying that
you "believe they are mathematically identical" immediately after opining
that there is a "mathematically provable difference" between them. I
suppose that's as good as any way of expressing confusion and bewilderment
after being exposed to the stormy sea of opinion on the topic :-)
It seems to me that there is very little difference between the actual
plurality runoff and the simulated one (IRV) in the case of a three way
race, because in either case there is just an initial choice (with no
ostensible information advantage in either case) and the other choice,
which is hardly subject to manipulation in either case because it is a
final head-to-head determination of the winner.
So in a three way race virtually all of the manipulation occurs before the
first vote, whether we use IRV or an actual runoff.
The main difference is this: in an actual runoff if one of the finalists
behaves badly (or even heroically) between initial vote and runoff vote,
you still have a chance to change your mind, which could be good,
depending on the accuracy of the new information (always a judgment call,
whether before or during the game).
I'm not sure what you are comparing IRV with in the case of four or more
candidates. On the one hand you refer to a two step runoff, and on the
other hand you talk about mathematical equivalence. Obviously a two step
plurality runoff cannot be mathematically equivalent to a plurality runoff
wherein several candidates are eliminated sequentially.
Since this more elaborate, many step, plurality runoff is the type that
IRV simulates, that is the type of "actual" runoff that I will compare IRV
to in the following comments.
If there are four or more candidates, there is a big difference in the
actual and the simulated.
In the actual runoff, the early results (suitably interpreted) serve as
fairly reliable polls to inform strategy for later votes. This advantage
is not available under IRV. IRV voters have to rely on the corporate
pollsters, news media, and other rumor mongers which may have a vested
interest in down playing the chances of a third party challenger, for
Under IRV all of the manipulation happens ahead of the game. There's no
chance to adjust and compensate for that later in the game, as more
reliable information surfaces.
Of course, some (superstitious) IRV proponents actually believe that their
best IRV strategy is to always rank the candidates in their sincere order
of preference, regardless of their chances of winning. For them, none of
my preceding comments about strategy and manipulation would make any
IRV may be more convenient than an actual runoff, but I don't see any
other advantage, mathematical or otherwise. And it seems to me that IRV
comes out on the short end of the stick strategically. So count me out of
any rumored consensus to the contrary :-)
Some of the other EM list members surely have insights that I've over
looked in this regard, or at least, pithier ways of expressing them.
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