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: http://en.wikipedia.org/wiki/Effects_of_different_voting_systems_under_similar_circumstances (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: http://en.wikipedia.org/wiki/Single-elimination_tournament 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 Seeding: #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 Tree: /-- #1 Memphis /------ Chattanooga | \-- #4 Chattanooga Nashville | /-- #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" Thoughts? Rob ---- election-methods mailing list - see http://electorama.com/em for list info
