James Gilmour wrote:
Kathy Dopp > Sent: Saturday, June 21, 2008 5:20 AM
"Later-No-Harm", however, is incompatible with the basic
principles of majority rule, which requires compromise if
decisions are to be made. That's because the peculiar design
of sequential elimination guarantees -- if a majority is not
required -- that a lower preference cannot harm a higher
preference, because the lower preferences are only considered
if a higher one is eliminated.
The meaning of the second sentence isn't completely clear to me, but I am
fairly sure there is a perverse interpretation of
"majority" in the first sentence. An IRV election is an Exhaustive Ballot
election contracted into one voting event, instead of
being spread over several rounds in which the one candidate with fewest votes
is eliminated at each round. It is no surprise that
the numbers of voters participating varies from round to round - usually a
progressive (or severe) decline. The votes in an
Exhaustive Ballot election might look like this:
To the degree that finding a good choice requires one to make a
compromise, and the method is supposed to be "as close to deliberation
as one can get", it would have to look at the entire ballot. Now you may
say that in real deliberation, as in a parliament, a participant doesn't
know of future choices of the others -- but it lets them change their
minds between each balloting, which no ranked method can do. The best a
ranked method can do is to use preferences to find something that can be
agreed by all, and for that, Kathy's "LNH incompatibility" argument holds.
Or more concrete: if you want the sort of compromise that Condorcet
gives (and you don't think that's a "weak centrist"), then you can't
have LNHarm. I don't think you can have LNHelp either, but I'm not sure
about that.
Round 1
A 4,000
B 3,000
C 2,000
D 1,000
Total voting 10,000
Round 2
A 3,500
B 2,500
C 1,500
Total voting 7,500
Round 3
A 3,000
B 2,000
Total voting 5,000.
A is the majority winner in Round 3, that is to say, the majority winner of
those voters then voting. And IRV satisfies that
criterion - and the Exhaustive Ballot is the valid comparison for IRV
(because that is the origin of IRV). The only difference is
that to ensure the integrity of the count (accounting for all ballot papers at
all stages of the count), the ballot papers (votes)
of those who opt out at the later stages (rounds) are recorded as
"non-transferable".
Any elimination method can have that criterion. As long as you don't
break early, after sufficient eliminations there'll be only two
candidates remaining. At that point, they're either tied or one of them
has a majority of those voters when voting. It doesn't matter if you use
Borda-elimination, IRV, average Plurality elimination (Carey's Q
method), or the exhaustive version of Coombs.
I seem to remember one on this list saying something to the effect of
"if you want to see how spurious this reasoning is, just take the
elimination process one step further and then you'll always have
unanimity! Except it isn't."
"Many" on this list may think that, but it is my experience of more than 45
years as a practical reformer explaining voting systems
to real electors, that 'later no harm' does matter greatly to ordinary
electors. If they think the voting system will not comply
with 'later no harm', their immediate reaction is to say "I'm not going to mark
a second or any further preference because that will
hurt my first choice candidate - the one I most want to see elected." And of
course, if you once depart from 'later no harm' you
open the way to all sorts of strategic voting that just cannot work in a 'later
no harm' IRV (or STV) public election with large
numbers of voters.
If the method fails LNHarm about as often as it fails LNHelp, then that
argument should fail, because bullet voting may harm your other choices
as much (or more, no way to know in general) as consistently voting all
of them will. Ceteris paribus, it's better to have a method that passes
both of the LNHs than neither (since you get strategy in the latter
case), but the hit you take might not be as serious as it seems at first.
----
Election-Methods mailing list - see http://electorama.com/em for list info
