Methods of elimination in quota preferential STV

[LAYTON Craig]

Bart Ingles wrote:

>True, if the voter gives her entire vote to a single candidate when that
>candidate was in no danger of losing.  But candidate elimination removes
>part of the incentive to vote for too few, in that you are less likely
>to see all your candidates lose because you spread your vote too thin
>(if your party is weaker than you thought, only one candidate would be
>eliminated & the votes redistributed, instead of eliminating most or all
>of them at once).
>
>So the problem should be less severe than with traditional limited vote
>or cumulative vote.

The problem is pretty severe, even when the popular candidate receives no
single votes (ie all those who vote for the popular candidate vote for
others).  Example;

An electorate is electing 5 members.  There are four parties.

Pary A has 40% support
Party B has 40% support
Party C has 11% support
Party D has 9% support

An election is held using your system.  Tiebreaker for two or more
candidates in the same party, is the candidate further down the party ticket
(ie with the higher number) is eliminated.  There won't be any ties between
parties so we don't have to worry about that.

Parties A and B stand 5 candidates each: A1-A5, and B1-B5.  Parties C and D
stand one each C1 & D1.  

Every A and B voter votes for 3 candidates each.  Why three? Why not? They
understand the electoral system enough to know that voting multiple times
can help their party, but they also want to help their preferred party
candidates win (after all, this is what individual candidate PR's all
about).

A1 is very popular, and every A voter votes for him, and two others (A2-A5
have an even spread).  B candidates have a perfectly even spread.  C and D
voters vote for their own candidates only.

The voting goes like this;
20% A1, A2, A3
20% A1, A4, A5
8% B1, B2, B3
8% B3, B4, B5
8% B2, B4, B5
8% B1, B5, B3
8% B4, B1, B2
11% C1
9% D1

I won't bother with the rounds of voting, but candidates are eliminated in
the following order;
A5, A3, B5, B2, D1, A4, A2.  

Candidates A1, B1, B3, B4, C1 are elected

Party A, with 40% support, gets 20% of the seats. Party B, with 40% support,
gets 60% of the seats, and party C, with 11% support, gets 20% of the seats.

This is a pretty dodgy result.  In fact, if it was a limited vote (Single
Non Transferrable Vote) with a similar voting pattern, A1, B1, B2, C1, D1
would be elected.  This is actually a much better result, even though it
isn't any more proportional.