Somebody took me to task about an hour ago in the comments section, sternly telling me "James, you don't understand the electoral system". He then proceeded to 'explain' the electoral system to me - but there was just one snag. His description bore absolutely no resemblance whatsoever to the Additional Member System, or indeed to any other voting system that is used in this country! I think possibly he'd "reverse-engineered" some of the modelling he'd seen on Wings and assumed there must be a fiendishly complicated formula involved.
It does seem to me that there's still an awful lot of confusion out there about how AMS works, so to put that theory to the test, here's a little quiz. There are only three questions, and the answers are at the bottom of the post. (No peeking in advance.) If you get all three right you are officially an AMS Grand Master.
"There is a cap on the total number of list seats any large party can win, regardless of how many list votes they take." Is this statement TRUE or FALSE?
"Small parties with enough votes are awarded a proportion of seats that is much larger than their proportion of the vote." Is this statement TRUE or FALSE?
If it was possible to successfully game the system by voting tactically for a small pro-indy party on the list, would that party be more likely to take a LARGER or SMALLER percentage of seats than its list vote would normally justify?
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Question 1. The statement is FALSE. If any party was to theoretically take 100% of the list vote, they would take every single list seat, and if they had swept the board in the constituencies as well, they would take 100% of the seats in the entire parliament. As with any proportional representation system, it's much harder to win a large number of seats - you're not going to win a landslide majority of seats on 35% of the vote, which is something that can easily happen under first-past-the-post. But there's no cap to directly prevent big landslides from occurring - you can win any number of seats providing you have enough list votes.
Question 2. The statement is FALSE. It's possible for a small party to get a slightly bigger proportion of seats than its proportion of votes, but the emphasis is on the word 'slightly', and there's certainly no in-built advantage for small parties. Quite the contrary, in fact - it's the largest single party that is most likely to be significantly over-represented, due to winning an excess number of constituency seats.
Question 3. The answer is SMALLER. This may seem counter-intuitive, but it's actually quite logical when you think about it. The whole game-the-system theory (which as you know I don't think is workable in practice) depends on the largest pro-indy party doing exceptionally well on constituency seats, but not being competitive at all on the list ballot. If that happens, there probably wouldn't be enough list seats available to bring any of the other parties up to quite the level of overall representation that their share of the list vote would warrant - and that applies just as much to the smaller pro-indy party that is the recipient of tactical votes as it does to all the other parties. So although the pro-indy parties in combination might end up being over-represented, the party that receives tactical votes might paradoxically end up being under-represented.