Accuracy of Polls in the 2021 Devolved and London Mayoral Elections

This May’s major elections were accompanied by a considerable amount of polling. This included a number of polls conducted close to polling day. Here we assess the accuracy of these final polls in Scotland, Wales and London.

We define as a “final poll” one that conducted at least some of its fieldwork on or after the Monday before the election.

Scottish Parliament

Voters were given two ballot papers – one for their local constituency MSP, the other for a party list. Five final polls were conducted and their estimates of both the constituency and list vote are shown below. The tables also compare the average of the polls with the eventual outcome.

Constituency Vote

 SNPConLabLib DemOtherMethodSample sizeFieldwork
YouGov52201963Online11442-4.5
Savanta ComRes42252283Online100130.4-4.5
Survation49212182Online100830.4-4.5
Ipsos MORI50202263Telephone150230.4-3.5
Opinium51231971Online101528.4-3.5
Average48.821.820.67.02.4   
Result47.721.921.66.91.9   
Difference+1.1-0.1-1.0+0.1+0.5   

The average estimate of the five polls is within a point of the final outcome for all of the parties, making it a very creditable overall performance.

List Vote

 SNPConLabLib DemGreenOther
YouGov3822165136
Savanta ComRes342319698
Survation3621197106
Ipsos MORI3923184124
Opinium412317685
Average37.622.417.85.610.45.8
Result40.323.518.05.18.15.0
Difference-2.7-1.1-0.2+0.5+2.3+0.8

The estimates of the list vote were slightly less accurate, with most polls underestimating SNP support and overestimating that of the Greens, but otherwise the average estimate was again no more than a point adrift of the final outcome.

The figures for two other polling companies whose last poll was completed before the Monday before polling day are shown below.

Constituency Vote

 SNPConLabLib DemOtherMethodSample sizeFieldwork
Panelbase48212074Online109628-30.4
BMG49192193Online102327-30.4

List Vote

 SNPConLabLib DemGreenOther
Panelbase392216788
BMG372217899

Welsh Senedd

Voters were again presented with both a constituency and a list ballot paper. Two final polls of voting intentions were conducted.

Constituency Vote

 Plaid CymruConLabLib DemOtherMethodSample sizeFieldwork
YouGov202936311Online11442-4.5
Savanta ComRes182836612Online100229.4-4.5
Average19.028.5364.511.5   
Result20.328.139.94.96.8   
Difference-1.3+0.4-3.9-0.4+4.7   

List Vote

 Plaid CymruConLabLib DemGreenAbolishOther
YouGov2125314576
Savanta ComRes1925325569
Average20.025.031.54.55.06.57.5
Result20.725.136.24.34.43.75.6
Difference-0.7+0.1-3.9-0.4+0.6+2.8+1.9

While very close for Plaid Cymru, the Conservatives and the Greens, both polls underestimated the level of Labour and Liberal Democrat support, and overestimated that for Others on the constituency ballot and the Abolish the Welsh Assembly Party in particular on the list.

London Mayor

Four final polls of the contest to be Mayor of London were conducted. The table shows their estimate of the first preference vote.

 ConLabLib DemGreenOtherMethodSample sizeFieldwork
Panelbase29408614Online10024-5.5
YouGov314351011Online11442-4.5
Savanta ComRes29418517Online100129.4-4.5
Opinium2948878Online100528.4-3.5
Average29.543.07.37.012.3   
Result35.340.04.47.812.5   
Difference-5.8+4.3.0+2.9-0.8-0.2   

These polls were less accurate than those in Scotland and Wales. Although the average figure for the Greens was almost spot on, as was the estimate for combined tally of minor parties and independents, the polls overestimated the Labour and Liberal Democrat vote, and heavily underestimated the Conservatives.