One of the arguments that's used against adopting Alternative Vote (AV) is that it isn't a proportional system, and in some cases is "less proportional" than First Past the Post (FPTP).
This argument annoys me not because it's false - it's entirely true - but because it's not relevant.
You can apply second-preference polling to the First Past the Post result fairly easily - the BBC has a chart of the 1983-2005 general elections and my alternative vote swingometer uses roughly the same calculations for its 2010 base case.
You can tell from this fairly easily - play around with the AV swingometer if you're unconvinced - that AV ends up more proportional if the winning party of Conservatives and Labour has less Lib Dem support than the second-placed party (as happened in 2010) and less proportional in the opposite situation (as happened in 1997).
This doesn't actually make a lot of sense, though.
Firstly, it's not sensible to assume that first preferences under AV are the same as votes cast under FPTP. You're essentially assuming no tactical voting if you do this, and while the extent and impact of tactical voting is heavily debated, no-one denies that it does occur. At the moment, polling suggests that 5-10% of voters do not cast a FPTP vote for their AV first preference, but the nature of tactical voting means that these may be more concentrated in the marginal constituencies.
It's difficult to model accurately. I'm not too bothered about it in the AV swingometer yet, because there's very little useful polling, and the boundary changes will mean that it's not particularly useful for predicting an election result, as opposed to looking at the general impact of particular preference distributions, at the moment. However, for talking about "what ifs" for previous elections, it matters.
People will vote differently under different voting systems, as the polling shows. Parties will campaign differently under different voting systems, because the proportion of the electorate that they really need to appeal to for a majority changes, and even which parties can exist with a reasonable chance to get elected changes.
Secondly, in a preferential system, the concept of proportionality starts to break down. Proportional to first preferences is usually what is meant, but that's not necessarily meaningful either because the preference ordering doesn't tell you about the gap between the preferences.
At the last election, had it been AV, I would have taken a long time to decide which order to put the first two preferences in, and it would have been - as it was under FPTP - a very marginal decision. The third to sixth preferences would have been very easy as there would have been a wide margin. So for proportionality my vote should probably have counted in the ratio 50:45:3:2:0:0, but someone else casting a vote with the same preference order might mean it as 70:20:5:2:2:1
Thirdly, neither is designed as a proportional system - it's not possible to have a proportional system while keeping a strict one MP per constituency and not having top-up MPs (this isn't quite true, actually1). Comparing AV and FPTP on proportionality, as well as depending on a number of false assumptions about voting habits and hidden definitions of proportionality itself, to me doesn't even make sense. It's like comparing two cars based on which one needs the shortest runway to land on.
A comparision between AV and FPTP should be based on the effect they will have at the constituency level, on campaigning methods and incentives, on the effect on political decisions, and so on. There may well still be consequences in that to make a supporter of proportional representation prefer FPTP over AV given that a PR system is not on offer (though that isn't my view).
1 Single-seat proportionality
There is actually one voting system which will usually (it's not actually guaranteed, but with the hundreds of constituencies the UK has, it's fairly likely) give a significantly more proportional result than FPTP using the existing constituency set up.
I call it "Vote From Hat". You take all the votes cast within a constituency, discard those for which voter intent cannot be determined or are otherwise invalid, and put the remainder into a giant hat. Stir well. The Returning Officer then selects at random a ballot paper from the hat, and declares the candidate chosen by that ballot paper to be the winner.
For a large enough number of seats this is almost as proportional as a whole-country list system would be, provided that there aren't huge turnout differentials or size differences between seats.
A quick review of the advantages of this system.
- Generally proportional on both a regional and national level. Regional parties (the Northern Ireland parties, the SNP, Plaid Cymru, Mebyon Kernow, etc.) are represented roughly proportionally within their regions without significantly affecting the national proportionality of the nation-wide parties.
- Maintains a strong constituency link as there is one MP per seat.
- Strong incentive to vote - your vote could literally be the one that makes the difference, in any constituency.
- Easy to get rid of unpopular MPs. Actually, quite easy to get rid of popular ones, too. There is no such thing as a safe seat.
- Very quick to count, and no need for recounts.
- No incentive for tactical voting - you should always vote for the candidate you genuinely want to win.
- Election night is much more exciting - even if the government comes out, as it did in 2001, with much the same number of seats that it started with, that doesn't mean that they'll be the same seats or that prominent ministers won't have lost their seat.
- Shows that proportionality is not the sole criterion by which a voting system should be judged.
Sadly it's not on offer.