HOW ACCURATE WERE THE OPINION POLLS?
May 13th, 2007For the last few weeks we covered the opinion polls in the run up to the election campaign. Now it is all over, bar the appointment of First Minister, just how accurate were they?
ScotlandVotes also publised a regular poll of polls and we will also take a look at how the final poll of polls faired against the real result on May 3rd.
The following is an edited excerpt is taken from the excellent UK Polling Report Blog by Anthony Wells. You can find it here.
With the results from the Scottish election in, how did the pollsters do? All the pollsters underestimated the level of Conservative support in the constituency section (and all by the same amount), Populus’s Labour support was too low and YouGov’s SNP support too high, leaving ICM as the most accurate in the constituency section.
Constituency Vote
RESULT YouGov ICM Populus
Conservative 16.6 -3.6 -3.6 -3.6
Labour 32.2 -1.2 -0.2 -3.2
Lib Dem 16.2 -2.2 -0.2 -1.2
SNP 32.9 4.1 1.1 -0.
Avg Error 2.8 1.7 2.0
Polling predictions were closer in the regional vote, with all three pollsters have an average error below 2% on the 4 main parties. Populus and ICM’s figures for the Liberal Democrats were both noticably high, but very accurate figures for the other parties meant Populus ended up with the figures closest to the actual result. YouGov were close behind, but got the balance of support amongst the smaller parties wrong, overestimating the level of support for the Greens - this is perhaps the result of switching to including all the minor parties in their question prompt for their last few polls. The final YouGov poll not to prompt using minor parties would have had an average error of 2.0% for the main 4 parties, and 1.6% if you included the Greens - suggesting it would have been less accurate on the main parties, but more accurate for the Green share of the vote. That said, it was done in the middle of April, so voting intention could have changed over the last couple of weeks of the campaign.
Regional Vote
RESULT YouGov ICM Populus
Conservative 13.9 -0.9 -0.9 0.1
Labour 29.2 -2.2 -0.2 -1.2
Lib Dem 11.3 -1.3 4.7 3.7
SNP 31.0 1.0 -1.0 0.0
Green 4.0 5.0 0.0 0.0
Average Error 1.4 1.7 1.3
Average Error (excld. Greens) 2.1 1.4 1.0
Poll of Polls Performance
Constituency Vote
Actual Poll of Polls Diff
Lab 32.2 31 -1.1
SNP 32.9 36 +3.1
Con 16.6 13 -3.6
Lib Dem 16.2 15 -1.2
Avg Error -0.7
Regional Vote
Actual Poll of Polls Diff
Lab 29.2 28 -1.2
SNP 31 32 +1
Con 13.9 13 -0.9
Lib Dem 11.3 13 +1.7
Green 4 5 +1
Avg Error +1.6

