[EM] Challenging the "instant runoff" name (and "instant playoff voting")

Rob Lanphier robla at robla.net
Mon Jun 19 23:08:58 PDT 2006

Hi all,

The reason why the naming of "instant runoff voting" is such appealing
marketing is because it paints a false picture.  I think people often
think they are getting the equivalent of a hierarchical tournament when
they advocate IRV.

Take the Tennessee example here:

(permalink to cited version:
http://en.wikipedia.org/w/index.php?title=Effects_of_different_voting_systems_under_similar_circumstances&oldid=57043174 )

To summarize:  given the choice between (from west to east) Memphis,
Nashville, Chattanooga, and Knoxville as candidates for the state
capital of Tennessee, a mock election (using fictional but credible
numbers) chooses Knoxville, even though Memphis is by far the most
populous, and Nashville is geographically central.

The reason why Knoxville can be the winner even though it loses to every
candidate but the Condorcet loser (Memphis) is because a real runoff
isn't happening.  Knoxville would be eliminated pairwise by any city
other than Memphis.  Since the only pairwise matchup is the final
matchup, and since that matchup is with the Condorcet loser, Knoxville
wins.  That's why IRV is only barely Condorcet loser compliant.

A real runoff would be a hierarchy of pairwise matchups, similar to a
single-elimation sports tournament with all of the same rules:

It occurs to me that a pretty cool voting system could be devised along
this path (and probably has been -- please let me know who I'm ripping
off here).  Seeding could be done by number of first place votes (i.e.
the plurality voting ranking).  So, for example:

Ballots (M=Memphis, N=Nashville, C=Chattanooga, K=Knoxville):
42: M>N>C>K
26: N>C>K>M
15: C>K>N>M
17: K>C>N>M

#1 Memphis (42 first place votes)
#2 Nashville (26 first place votes)
#3 Chattanooga (17 first place votes)
#4 Knoxville (15 first place votes)

First round:
#1 Memphis vs. #4 Chattanooga
Chattanooga beats Memphis 58-42 in a pairwise election

#2 Nashville vs. #3 Knoxville
Nashville beats Knoxville 68-32 pairwise

Second round:
#2 Nashville vs. #4 Chattanooga
Nashville beats Chattanooga 68-32 pairwise

           /-- #1  Memphis
   /------ Chattanooga
   |       \-- #4  Chattanooga
   |       /-- #3  Knoxville
   \------ Nashville
           \-- #2  Nashville

This system has a very similar appeal to Copeland: it's Condorcet winner
compliant and it's easy to explain to sports fans.  It has the added
advantage that ties are much harder than in Copeland, since not all
pairwise comparisons are considered.

I wouldn't be surprised if there are grave problems with this system in
the event that there's no single Condorcet winner, but I haven't worked
out what those problems would be.

Regardless of whatever merit it may or may not have as a voting system,
it seems clear to me that this system is more deserving of the "instant
runoff" name than what is currently marketed as IRV. In the interest of
giving it a distinct name for discussion purposes, I'll call it "instant
playoff voting"


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