Sunday, 27 March 2011

The Alternative Vote #2: How does it work?

If you've voted before, the you'll know that under the present First-past-the-post system you can select one candidate from the ballot paper to represent your constituency. The number of votes for each candidate are counted, and the winner is the candidate with the most votes. Note that this means that a candidate can be elected irrespective of whether he/she has the majority of the support. 

Under AV, you're asked to rank the candidates in order, where 1 is highest. (In the proposed UK system, you don't have to rank every candidate.) The first step in determining a winner from everyone's rankings is to count all of the first preferences for each candidate. If one candidate has more than half of the first preferences, then he/she is elected. If no candidate has a simple majority, however, then the candidate with the least first preference votes is eliminated. The preferences of those voters who most preferred the eliminated candidate are not discarded though; their second preferences are taken into account, and are distributed to the remaining candidates. This procedure continues, with progressive elimination of the candidate with least support until a candidate obtains at least 50% of the votes.

I've made an example to reinforce the idea ...

First-past-the-post ballot paper

Alternative Vote ballot paper

An example

There are 4 candidates: A, B, C, D and 100 voters. 
The voters make their ranking and the ballot papers are collected. What happens next?

1) The first preferences for each candidate are counted and the results are A: 40  B: 30  C: 20  D:10.
Since no candidate has a majority, the candidate with the least first preferences, D, is eliminated. The votes of people for whom D was favourite are not eliminated though; their vote is re-allocated to the remaining candidate that they ranked 2nd favourite. 

2.) After the redistribution of D's votes, the results are A: 42   B: 35   C: 23.
Still, no candidate has more than 50% of the votes. C is eliminated, and the second preferences of those who most preferred C are distributed to A and B. If ones of these voters placed D in second place, then their third ranking is considered. 

3) After the redistribution of C's votes, the results are  A: 46   B: 54.
B has the majority of votes and is the winning candidate.

Note that B has won despite being fewer peoples first choice. AV works to identify the candidate that most electors would like as their representative if they cannot have their first choice because he/she doesn't have a majority support.


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