STV is a constituency system but a multi member one (STV in single member constituencies is called AV and is a completely different system, because the Politicians it elects are not proportional to the votes the party receives, and they are usually Australian. The fewer constituencies you divide your population into the more proportional STV will usually be, but the ballot papers can get rather long and very difficult to count.
Unlike list systems of proportional representation which encourage small but tightly centralised parties, STV tends to encourage large but decentralised parties. This is because politicians with a strong personal vote are a big asset to any party in STV, and larger parties will tend to put up multiple candidates in each constituency where they hope to win more than one seat. Those candidates are campaigning both with and against each other. Also multi member systems tend to underrepresent small and evenly spread parties.
For voters it is a very simple system, you get one ballot paper with a long list of candidates on it and you put them in order 1 to whatever, marking as many choices as you care to. You can choose to put all the candidates of your favourite party before any other party, or you can mix and match, putting the candidates in any sequence you wish. That does mean that sometimes an election will produce the same party balance but a whole bunch of seats will change hands within parties on an issue that matters to the voters but cuts across party lines.
STV lends itself to deal making between and within parties. Imagine an election where the Greens and Lib Dems were each asking their supporters to give their second preference to the other party, and UKIP were being asked by their voters whether they were endorsing the Tories or the BNP for second preferences. Such deals whether tacit or otherwise do work, but its up to the voters. In Northern Ireland preferences largely move either amongst the Unionist parties or the nationalist ones, whether there is a deal or not.
Counting STV elections is fiendishly complicated and best done by use of sophisticated computer programs. In principle it is as easy as:
- count how many people have voted in total, that's your number of votes cast (you may at this point remove any spoilt or blank ballots as otherwise they will be a complete pain later)
- calculate the quota. That's the number of votes cast divided by one over the number of people you are going to elect plus one; plus one. So if 200,000 people voted and you are going to elect three people, then the quota is 200,000 divided by four, plus one which gives you a quota of 50,001. This may sound illogical but once three people have had 50,001 votes out of 200,000 there are less than 50,001 left.
- Sort the ballot papers into piles according to people's first choice vote, and count each pile.
- Any candidate who has received more votes than the quota has now been elected.
- Subtract the quota from the number of votes each winning candidate received, the difference is that candidates surplus. If you are incredibly lucky this will be nice and simple, the candidate who came first got 100,002 first preference votes, only needed 50,001 to be elected and therefore had a surplus of 50,001 so each of their 100,002 votes is now worth half a vote to whoever was that voters next choice. More realistically you can expect to have 55,000 votes each worth less than 0.1 of a vote and 51,000 votes each worth less than 0.02 of a vote.
- If you haven't yet elected as many candidates as you intended to, you can now start eliminating the least popular candidates and distributing surplus votes, starting with the smallest vote or surplus, and stopping each time a candidate reaches the quota.
- Each time you eliminate a candidate you need to sort their ballot papers into piles for the candidate that is their next preference, ignoring candidates who have been eliminated or elected. Then recalculate the number of vote for each of the remaining candidates, see if anyone else has been elected and then eliminate whoever is now last.
- When you are transferring the surplus votes of an elected candidate you (or hopefully your computer) needs to remember that this is only a fraction of a vote, and if you are really unlucky a fraction of a fraction of a vote.
In practice and with five or more seats in a constituency it can get significantly more complex than that. In a first past the post election you can win by as little as one vote or even get a tie and have to draw lots. With STV people have been elected with majorities of 0.02 of a vote.
Even in a very simple example such as the European elections in Northern Ireland, ark.ac.uk where two candidates are elected over the quota, several candidates eliminated and their votes transferred according to the voters preferences between the competitors for the third seat. you still have to look at the surpluses from the elected candidates to decide the final seat.
STV is a proportional system that also has constituencies to link politicians with particular parts of the state or country, it is simple for voters, gives voters a choice between different factions within parties and benefits politicians and parties that are peoples second or third choice as opposed to parties such as fascists that are loathed by everyone except their supporters.
Um how do you count these elections? and what do you do if an elected politician dies?