Abd ul-Rahman Lomax wrote:
I took a preliminary look at the November 2008 San Francisco RCV elections.
There were 7 Supervisors elected. 3 elections found a majority in the
first round.
In the remaining four, no majority was found after transfers, I haven't
looked at all of them in detail, but one was on the order of 40%.
In no case did the first preference leader fail to win.
This is seems to be typical of nonpartisan IRV. So far, no comeback
elections, excepting a partisan election in Pierce County, Washington,
out of what must be approaching forty elections.
This appears to be normal: most of the time, when a majority isn't found
in the first round, no majority is found after vote transfers (except
for the "last round majority," which isn't a majority of the votes and
is a mathematical certainty from the method, which essentially discards
all votes which aren't for the top two).
Further, in nonpartisan elections, the preference order in the first
round tends to be maintained after transfers. No reversals have been
seen. In Top Two Runoff elections, reversal takes place roughly
one-third of the time.
I have the ballot image files for the "instant runoffs" from San
Francisco. Rather irritatingly, they don't have the files available from
the districts where there was a first round majority; but those results
might be in the overall result file that I also have, I haven't looked
at it yet.
San Francisco could have replaced their prior TTR system with Plurality;
the results would have remained the same. However, previously, there
were comeback elections where the runner-up did win. IRV eliminates
that, in nonpartisan elections, almost entirely. I say "almost" not
because I've seen an exception, but because the vote gaps do reduce,
sometimes, and in a very close election, a "comeback" could occur. It's
clearly rare with IRV.
I've run that data through my program as well. In all of the districts,
the Plurality winner would have won using Schulze or Minmax. In all but
one of the district, the same candidate would have won no matter which
of the methods I tried were used. I had some trouble with the RCV
truncation as far as regards Vote for and Against and Antiplurality, so
I left them out.
There are some interesting patterns in the ballot data and the official
interpretation. For instance, some people effectively bullet voted by
voting the same candidate for first, second, and third place. Others
voted A > A > B in order to vote A > B (and nothing else beyond that).
Also, it seems that the official IRV count stops counting a ballot when
an overvote has been detected at that rank, but simply skips the rank if
there's an undervote (no vote at the rank). For instance, A > B = C > D
would count as a bullet vote for A, whereas A > (empty) > B is counted
as A > B.
In my opinion, the restriction to three ranks worsened the results
considerably. For instance, in district 9, 2398 votes didn't get counted
at all. That's nearly a tenth (8% to be precise).
Oddly, there seems to be just about the same level of bullet-voting as
in Burlington (about 25%), at least from my quick check of District 1.
The results are at
http://munsterhjelm.no/km/elections/SF2008_results.txt and the ballots,
in [number]: A > B > C format, is at
http://munsterhjelm.no/km/elections/sf2008.zip . Note that the ballot
files have one entry of the form [number]: each, with no ranking
following - that's the count of invalidated ballots.
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