A while ago I posted about voting tactically in the European Elections. The Scottish Parliament elections are coming up this week. I thought I'd explain how, though the electoral system is similar, there's an important quirk that affects tactical voting.

As in the European Elections, when you vote for a list you give one vote to the first person on the list, half a vote to the second, a third of a vote to the third, and so on. However, the list you vote for might not be the list that's on your ballot paper.

Why? Because there are also single-member constituencies. Every candidate that wins a constituency gets added to the top of their party's list.

So, for example, here's are the results for the Lothian region in 2011, when the SNP won 8 constituencies and Labour won 1. Lothian elects 16 MSPs, so I've numbered the 16 candidates with the most votes.

There are two things to take note of here. Firstly, though they've been demoted, the SNP's Somerville and Orr weren't actually at the top of the SNP list. The candidates who were ahead of them were also running for constituencies which they won.

Secondly, I've shown the candidates who got elected in bold. You'll notice that this doesn't correspond to the top 16 candidates. When a party wins more seats via constituencies than they're entitled to by the list vote, another party (or parties) lose(s) out.

This is where tactical voting comes in. If a party is winning so many constituencies that it's exceeding its allocation, then there's no point voting for its list. The result for the Lothian region would have been exactly the same if nobody had voted for the SNP's list. With each vote counting for a ninth of a vote, the SNP would have needed 37,633 more votes to win an extra seat (by defeating the Conservative Gavin Brown). In contrast, Labour would have needed just 12,004 more, while the Liberal Democrats (who got 15,588) would have needed a mere 922 more.