How It Gives Proportional Representation
STV is proportional for every characteristic the voters value. e.g. a portion of voters ranking all women first will result in this portion rounded down to next quota will of women at least represented.
It ensures proportional representation simply by wasting few votes.
A vote is wasted if it does not elect anyone; it is partially wasted if it elects someone who gets more votes than is necessary to be elected. STV transfers votes that would otherwise be wasted, and it only transfers such votes.
However, the degree of proportionality achieved is strongly related to the district magnitude, or the number of seats that are to be filled at any election. For example, under the Droop quota in a three-seat district, one vote less than a quarter of the total number of votes may not directly contribute to the election of a representative. Therefore, a desire for a high degree of proportionality is best support by large district magnitude.
It can often give non-proportional results in close elections such as the 1981 election in Malta where the Maltese Labour Party won a majority of seats despite the Nationalist Party winning a majority of votes. This caused a constitutional crisis, leading to provision for the possibilty of bonus seats to ensure proportionality, as proved necessary in 1987 and 1996. Similarly, the Northern Ireland elections in 1998 led to the Ulster Unionists winning more seats than the SDLP, despite winning a smaller share of the vote.
Potential for Tactical Voting
The single transferable vote eliminates much of the reason for tactical voting.
A voter is "safe" voting for a candidate they fear won't be elected,
because their vote will be reallocate in Process B. They are "safe" voting for a candidate they believe will receive overwhelming support, because their vote
will get reallocated in Process A.
However, there are loopholes:
candidates who have already been elected do not receive any more
votes, so there is incentive to avoid voting for your top-ranked
candidate until after they have already been elected. For example, a
voter might make a tactical decision to rank their top-place candidate
beneath a candidate they know will lose (perhaps a fictional
candidate). If the voter's true top-place candidate has not been
elected by the time their fake top candidate loses, the voter's full
vote will count for their true top-place candidate. Otherwise, the
voter will have avoided having had their ballot in the lottery to be
"wasted" on their top-ranked candidate, and will continue on to
lower-ranked candidates.
A vote is wasted if it ends up on the last candidate to be eliminated.
See also:
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