[EM] IRV vs Plurality

Jameson Quinn jameson.quinn at gmail.com
Sun Jan 10 12:08:55 PST 2010


>
>
> However, there are forms of Range voting which would avoid this problem.
> Most obviously, there is Approval itself, which is technically a form of
> Range. That can be extended to any version of Range which disallows or
> strongly discourages most "weak" votes. For instance, there could be options
> to vote only 0 ("worst"), 1("bad"), 50 ("intriguing"), 99 ("better"), and
> 100 ("best"). 50 is labeled as "intriguing" because the option is intended
> particularly for little-known candidates whom a voter believes, but is not
> sure, deserve support, not for well-known candidates who are simply of
> average quality. This would give additional expressiveness beyond Approval,
> without a significant risk of pathological situations where a strategic
> minority imposed their will over an unstrategic majority. For instance: in a
> standard "two frontrunner" scenario, where no voter was uninformed about
> either of the frontrunners and so nobody rated either as merely
> "intriguing", such a system could not give the "wrong" strategic-plurality
> winner unless the gap were less than 1% (ie, closer than 49.5%-50.5% of the
> two-way vote).
>
>
> In practice it might however make sense to rate the two frontrunners
> strictly at 0 and 100. Elections are often tight and therefore also smaller
> differences than 49.5%-50.5% may be crucial / quite often decide which one
> of the frontrunners will win. This approach would mean that all other
> ratings (1, 50, 99 and others) would be reserved for giving sincere opinions
> on candidates that can not win.
>

In practice, that is exactly how many people would vote. The "49.5% winner"
scenario is just theoretical - the worst practical result imaginable would
probably be a "49.93% winner" who actually deserves to win.

Why? Even when a "frontrunner" got a strong "third party" challenge, that
would only cost them half a percent of margin TIMES the percent of voters
voting "unstrategically". To actually win with 49.5-50.5 would take a
perfect storm - two mutually-incompatible third party challengers to the
50.5% (otherwise, one of them would win); a worse-than-bad candidate with a
plausible threat of winning (whose 0 could thus push voters to give the 49.5
candidate 1's); and 50.5% purely nonstrategic voters facing 49.5% purely
strategic voters. That's at least 4 factors, combined; in practice, you
might at worst occasionally have 1 of these factors strongly and the others
at 50%, which would mean that the margin "lost by the system" would be under
0.15% - truly negligible. And in fact, these factors tend to reflect badly
on the candidate who loses thereby, or well on the candidate who wins -
enough so, so that it is in fact more likely that this "pathology" would
actually help elect the socially-optimal candidate.

Jameson Quinn
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