Prediction errors are caused by two different types of effect: opinion poll error and model error, giving the total error.
In summary, 32 seats were incorrect due to opinion poll bias, and 27 seats were incorrect due to model error. But in total only 35 seats were incorrect because in 12 seats the two errors cancelled each other out.
At the next election about 50 seats will be incorrect.
The causes of opinion poll error include:
This figure is calculated by comparing the prediction with two different sets of inputs. The first uses the average of the final opinion polls of the campaign. Those were: Con 30.3%, Lab 46.3%, Lib 17.5%. The second uses the actual support levels in the election of Con 32.7%, Lab 42.0%, Lib 18.8%. Seats which have different predictions under the two sets of inputs are seats predicted incorrectly due to opinion poll error.
In total, opinion poll error understated the number of Conservative seats by 32, overstated Labour by 29, and overstated the Liberal Democrats by 3.
|Altrincham and Sale West||LAB||1.1%||CON||5.6%|
|Bedfordshire South West||LAB||3.7%||CON||3.0%|
|Beverley and Holderness||LAB||2.4%||CON||4.3%|
|Boston and Skegness||LAB||2.6%||CON||4.1%|
|Bury St Edmunds||LAB||3.3%||CON||3.4%|
|Dorset Mid and Poole North||LIB||0.5%||CON||0.6%|
|Lancaster and Wyre||LAB||6.2%||CON||0.5%|
|Milton Keynes North East||LAB||4.4%||CON||2.3%|
|Norfolk North West||LAB||6.3%||CON||0.4%|
|Rugby and Kenilworth||LAB||4.8%||CON||1.9%|
Since the Conservatives were underestimated by the opinion polls, all these seats are seats the Conservatives should have been predicted to win, but were not.
Predictions are shown as they would have been, had the correct national support percentages been used. That is Con 32.7%, Lab 42.0%, Lib 18.8%. No tactical voting adjustment was used to make the prediction.
In all, 27 seats were incorrect. The net effect was that the Conservatives won 11 seats fewer than predicted, Labour won 7 more, the LibDems won 5 more, and the nationalists (SNP) were 1 seat worse off. The fact that the errors are not symmetric suggests that Labour and Liberal Democrat supports used tactical voting against the Conservatives.
The table below shows the 27 seats, with the predicted winner and majority against the actual results. Important local factors are included as a comment, if applicable.
|Bristol West||CON||0.3%||LAB||8.0%||Double error|
|Chesterfield||LAB||7.8%||LIB||5.8%||Tony Benn stands down from seat|
|Dorset Mid and Poole North||CON||0.6%||LIB||0.9%||Double error|
|Dorset South||CON||2.9%||LAB||0.3%||Double error|
|Harrow West||CON||0.4%||LAB||13.2%||Double error|
|Isle of Wight||LIB||9.5%||CON||4.5%|
|Lancaster and Wyre||CON||0.5%||LAB||0.9%||Double error|
|Milton Keynes North East||CON||2.3%||LAB||3.9%||Double error|
|Northampton South||CON||1.4%||LAB||1.7%||Double error|
|Romsey||CON||15.9%||LIB||4.9%||LibDems consolidate by-election gain|
|Rugby and Kenilworth||CON||1.9%||LAB||5.3%||Double error|
|Tatton||MIN||22.1%||CON||20.9%||Sitting independent MP Martin Bell stands down|
|Wyre Forest||LAB||9.9%||MIN||35.9%||Strong independent candidate wins seat|
|Carmarthen East and Dinefwr||LAB||6.3%||NAT||6.8%|
|Galloway and Upper Nithsdale||NAT||12.6%||CON||0.2%|
|Opinion poll error||32||+32||-29||-3||0|
Paradoxically, the total number of seats mis-predicted (35) is much less than than the sum of the opinion poll and model errors (59). This is because twelve seats suffered mis-predictions in opposite directions by the two errors. For instance, Braintree was predicted to be a Labour hold, which was what actually happened. The prediction's accuracy was specious however. If the opinion polls had been correct, it would have been predicted as a Conservative gain. But the model was as wrong as the opinion polls, so Labour held it (just).
The 2001 prediction result is thus a little less accurate than it appears.
It should be assumed that about 50 seats will be mis-predicted at the next election.