There’s a sudden revival of interest in STV as the LP adopts it as a means of electing the CLP reps. There are in my mind, three problems with STV for party managers and one for voters. The fact that “order” can be critical in the results is important. When combined with the degree of discipline within Party voting blocs and the propensity to bleed votes from the block, getting the quota early is of advantage to parties/slates. The problem of avoiding having large numbers of your votes trapped in the losing quota is also critically important for party/slate managers particularly if the quota is high.
The Order based nature of STV counting leads to a series of well-known problems with STV in that it is one of the easier systems to game and has a number of design features which encourage manipulative or gaming behaviour. Voting systems have properties, categorised by academic writing. STV is not monotone, participative, consistent nor does it meet the No-Betrayer criteria.
There is no strategy that can compensate for a bloc’s voters not voting for all the candidates in the slate. Even within the activist or membership layer, many are more committed to their faction than to the Party, which may lead people to desert their first choice slate for another as the individual candidates become distasteful to the voter. I am curious if we could model the effect this ‘bleed’. The speed of desertion may well be determined by the slate construction as some on the slate might be exceptionally unacceptable to the otherwise loyal electorate. We should also note that not transferring reduces the quota[1] and so is equivalent to a (part) vote for all remaining candidates and acts as de-facto vote in favour of the highest remaining contender.
Early success militates against having votes lost in the losing quota, particularly as the STV transfers surpluses before eliminating candidates.
If running a complete slate, the offer of a recommendation to support as a second choice is not worth so much.
The disadvantage to voters is that their optimal strategy may not involve voting in a straightforward way as it may be best if someone likely to pick up a lot of transfers is eliminated before the transfers occur. This is more acute in instant run-off elections and depends on whether they want a candidate to win or prefer that others lose.
I can’t see what to do about weak discipline. If we take the example of the UK where we have Labour, the Greens, the Lib Dems and the Tories, we can see how people, who’s first choice might be, say, Labour might switch to the Green list after one or two votes for what ever reason and the Greens might go in both directions to Labour or to the LibDems. The system is designed to have this effect.
[1] This is the failure to meet the Participation Effect.