A UK-wide referendum is scheduled for 5th May 2011 to determine whether there should be a change in the voting system for the UK (Westminster) Parliament. The choice will be between the current First Past the Post (FPTP) system, in which you give an ‘X’ to each listed candidate in a single round of voting and the candidate with the most ‘X’s is elected, and the Alternative Vote (AV) system in which you can express preferences for each listed candidate by giving them a number from 1 downwards.
I think the easiest way to understand the Alternative Vote (AV) is to think of it as a multi-round elimination election, in which you specify in advance on one ballot paper how you would vote in each round.
In the first round of counting, instead of the candidate with the least ‘X’s being eliminated, the one with the least ‘1’s is. Then instead of asking everyone to vote again, the ‘2’s of those giving the eliminated candidate their ‘1’s are appropriately transferred to the remaining candidates. The bottom candidate is again eliminated and the same process carried out with their ‘3’s. And so on until one candidate has 50% of the ballots.
The last round is equivalent to a round in which, while some candidates have been eliminated, everyone has a vote between those remaining and the one getting an absolute majority of those votes is elected. So there’s no reason to say that second or third etc preferences should somehow have less value.
I’ve written some more about AV for the UK here.