Using the results of the 2011 Central Scotland Region to look at how tactical voting on the list effects the result in Scottish Parliamentary elections.
The Central Scotland region has nine constituency seats and seven list seats. The results in 2011 for the constituency seats were:
- SNP: 6
- Labour: 3
Regional list votes numbers:
- SNP: 108,261
- Labour: 82,459
- Conservative: 14,870
- Green: 5,634
- Lib Dem: 3,318
The regional list seats are allocated using an additional member system. The resulting seat allocation was:
- SNP 9 (6 constituency + 3 list seats)
- Labour 6 (3 constituency + 3 list seats)
- Conservative 1 (1 list seat).
We will now look at tactical voting by SNP voters who give their list vote to the Greens. This assumes that the constituency voting is unchanged.
With a 5% switch, the SNP have lost a seat and Labour have gained a seat. A gain of one for Unionist parties.
With a 10% switch, the SNP have lost a seat and the Greens have gained a seat. No change for Independence/Unionist seat ratio.
With a 15% switch, the SNP have lost 2 seats, and both Labour and the Greens have gained a seat. A gain of one for Unionist parties.
With a 20% switch, the SNP have lost two list seats to the Greens. No change for Independence/Unionist ratio.
With a 25% switch, the SNP have lost all of their list seats. Labour gain two, and the Greens gain one. A gain of one for Unionist parties.
With a 30% switch, the SNP lose all their list seats to the Greens. No change for the Independence/Unionist ratio. The results are the same for switches of 35%, 40%, and 45%.
With a 50% switch, the SNP lose all their list seats, Labour lose one, and the Greens gain four. A gain of one for Independence parties. Finally, at a 50% switch, do Independence parties gain from tactical voting! The results are the same for switches 55% and 60%.
With a 65% switch, the Conservatives lose their only list seat to the Greens! Maybe it’s all worthwhile – if only you could persuade 65% of SNP list voters to switch to the Greens. The result remains the same for switches of 70%, 75%, 80%, and 85%.
With a 90%, 95%, and 100% switch, the Greens take another list seat from Labour.
In summary, it is probably safe to assume that you will not be able to persuade a large percentage of SNP voters to switch their list vote to another party. It is then possible that Unionist parties will gain from a small switch.