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I spent Easter weekend in Northern Ireland and was intrigued by the posters for the Stormont Assembly elections, which all encouraged people to number their preferences. Over there they use Single Transferable Vote (STV) in multi-member constituencies - so the ballot paper would be rather like the ones we get here for local elections, with three names from each of the larger parties plus some independents. Voters rank them 1, 2, 3, 4 etc instead of by using a maximum of three crosses. (Incidently, it was absolutely essential to get proportional representation at Stormont, which is why they used STV and not First Past The Post).
STV and AV are both preferential voting systems, in that the voter numbers the candidates in order of preference. AV can be used in constituencies where there is only one winner, whereas STV is more suited to multi-member constituencies, such as local council wards.
From some of the debate about the referendum you might imagine that preferential systems are totally alien to British politics. In fact, they are already widely used.
Next year for the election of the Mayor of London, we will use a version of AV with just two preferences.
In Scotland they use STV for local council elections. Wales in considering doing the same in the future.
And, as mentioned before, the Assembly Members in Northern Ireland are elected by STV. They also use STV for elections to the European Parliament.
You probably hadn't noticed, but intriguingly, there are two ballots going on in the House of Lords at the moment, and they both use AV. One is to fill a vacancy in the small number of remaining hereditary peers, and the other is for the Speaker in the House of Lords.
And to cap it all, the leaders of the Labour and Conservative parties are both elected using a variation of AV - indeed, if FPTP had been used for the Conservative leadership, David Davis would now be Prime Minister, not David Cameron, who trailed behind on first preferences.
Odd, isn't it, that Parliament uses AV for its own elections and yet a sizable number of Parliamentarians are trying to tell us that it is not suitable for the rest of us!