Some form of proportional representation may be on the cards if any form of deal is done between the LibDems and another party to form a coalition government. But what form of PR? The one most commonly bandied about by supporters of PR is the Single Transferrable Vote, or STV, used to elect multiple members from larger constituencies.

STV works fine for relatively small electorates with a reasonably limited number of candidates, particularly where there are no real “party” distinctions. But it has a number of weaknesses which, in my opinion, make it unsuitable for electing a Westminster-style legislature.

Firstly, electing a multi-member consituency weakens the link between an MP and his/her constituency by making it a many-to-many relationship instead of one-to-many. That may not seem much of a problem, but it represents a fundamental change to the concept by which our parliamentary process operates – it wouldn’t be just a minor tweak.

Secondly, STV encourages the belief that you can only be represented by someone who shares your political position. At the moment, we have the principle that an MP represent every resident of the constituency, whether they voted for him/her or not. And my experience is that, for the most part, MPs do take that responsibility seriously – it’s certainly true that someone with a reputation as a “good constituency MP” is more likely to be re-elected, irrespective of their political colour, and that’s a good thing. But if a single constituency has MPs from two or more parties, then the natural tendency is to assume that those from one party represent people who voted for that party, while those from anotherparty represent their respective voters. That’s fine if you voted for one of those elected, but what if you’re not? In my own part of the world, if we’d had STV with, say, three members being returned, then we’d probably have two Conservatives and one LibDem. But who, then, represents the Labour, UKIP or Green voters?

Also, STV tends to lead to unwieldy ballot papers. Again, in my constituency, we had five candidates. If this were a larger constituency returning three MPs via STV, then we’d have at least fifteen, and probably more. How many is too many? (And this, incidentally, is why we don’t have STV for the European elections – despite the fact that party lists are widely unpopular, the size of the constituencies and the number of parties contesting them makes STV entirely impractical). In Ireland, where STV is used and ballot papers are, typically, very long, one observable outcome is that candidates with names beginning with earlier letters of the alphabet are significantly more likely to be elected.

Another issue with STV is how you handle by-elections. Do you re-run the whole election, including the places for the members who haven’t died or retired, or do you just have an election for the one vacant space? The former is somewhat impractical, especially if one of the other MPs happens to be a government minister, the latter will tend to make the result non-proportional – if you have, say, two Conservatives plus a LibDem, and the LibDem dies, then a one-place by-election will almost certainly return a Tory.

A major disadvantage with STV is that the method of counting it is complex. There are, in fact, multiple different methods of counting STV, and in close elections they can give different results. Close results also depend to some extent on purely random factors, such as which ballot papers happened to be counted first. That has the potention for accusations of deliberate manipulation and unfairness, and, even if these accusations are unfounded, they are very difficult to disprove. So STV, even more than the current system, requires a great deal of trust by the electorate in those who manage the process.

Finally, STV is very susceptible to tactical voting. But, given the complexity of the way that STV is tallied, those tactics aren’t immediately obvious to the average voter. That gives an advantage to the STV-savvy voters who are more likely to get what they want than those who simply cast their vote according to their honest preferences.

So I don’t want STV. I do want a reformed Parliamentary system. But adopting STV for Commons elections is not the solution.