Labour leadership election - How the Alternative vote works

As we close in on the date (September 12th) when the new Labour leader will be announced, those of you who may not have learned election systems yet might be wondering why, if more people vote for Jeremy Corbyn than any other candidate, he still may not win. The answer is that instead of using a First-Past-the-Post (FPTP) like "simple plurality" system, where each voter gets one vote for a candidate and the candidate with the most votes wins, regardless of whether they have a majority, Labour uses an "alternative vote" (AV) system.

AV is a "majoritan" election system, which means that the candidate that wins gets 50% of the votes. That is achieved by the voters ranking candidates, in this case four, making their favourite their first preference and, should they not win, they can choose to state a second, third and fourth preference. They don't have to rank all the candidates (for instance, Gordon Brown didn't put a preference by Jeremy Corbyn's name), but if they don't then their ballot paper may have less impact, because every one of their preferences might count.

That said, it is possible nothing but first preferences will be needed - because if over 50% of the voters make Jeremy Corbyn their first preference, he will win after the first round of counting. This is entirely possible, although it is more likely that it will take a bit more time than that.

According to sources, Yvette Cooper's team are counting on Jeremy Corbyn getting fewer than 43% of the first preference votes. Let's say that happens, and at the end of the first round of counting first preferences, the candidates have the following vote shares:

Corbyn 42%, Cooper 33%, Burnham 19%, Kendall 6%

Liz Kendall would be eliminated from the contest, and those that voted for her with their first preferences will have their second preferences looked at, and those allocated to the other candidates (unless they didn't use their second preference, in which case that ballot paper won't be used). Kendall represents the right wing of the party, and it is reckoned that most of her second preference votes would be for Yvette Cooper, given Andy Burnham is trying to represent the left-wing of the party and Jeremy Corbyn is actually representing the left-wing of the party. However, we could assume that Cooper will get 5% and Burnham 1% added to their tallies, with Corbyn getting nothing, leaving the candidates with the following vote shares:

Corbyn 42%, Cooper 38%, Burnham 20%

Now is the moment of truth. Andy Burnham is eliminated, and it is what those who put him as their first preference have put as their second preference than now matters. Should they have put Liz Kendall as second preference by the way, then their third preference would be counted. Again, should those who voted for Andy Burnham not used their second preference vote, or used it for Liz Kendall and not used their third preference vote, their ballot paper would be discarded.

Put simply, Yvette Cooper would need for over 60% of Andy Burnham's supporters (12% of total voters) to have made her their second preference to win, and Jeremy Corbyn would need 40% of those supporters (8% of total voters) to have made him their second preference to win. One of them will have over 50% after that third round.

Put like this, and you can begin to see how important it could be for Jeremy Corbyn to have as many first preference votes as possible. Otherwise, he is depending on Andy Burnham to be eliminated, and for those who voted for him with their first preference to have decided to put Corbyn as their second preference.

Will that happen? We'll know by this time next week.

p.s. It may be that Andy Burnham is second after the second round, in which case the same applies, just with 'Burnham' instead of 'Cooper' - but the bookies don't think it will be this way round!