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Your regional list vote –
The general rule here is that you vote for your preferred party since in most parts of Scotland most of the main parties (including the Greens) have a good chance of winning one or more regional list seats. To help you decide which party you prefer, see 2 above.
The only exception – and it is contentious – is that in a region such as Lothian where it is widely expected that the SNP will win every constituency seat (or, possibly, all but one seats) the SNP will then have no hope of winning any regional seats because of the way the regional list formula works. This argument is put forward strongly in section 3 of a report for the Electoral Reform Society by election expert Prof John Curtice. If you accept his argument, then a vote for the SNP on the regional list will have no effect, so even if you support the SNP you are better to vote for your next most favourite party. The main argument against is that if the polls are badly wrong, and the SNP miss out on 2 or 3 constituency seats, then the formula would give them a chance to pick up a regional list seat.
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http://www.spokes.org.uk/2016/04/election-who-shall-i-vote-for/