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

	SNP	Con	Lab	Lib Dem	Other	Method	Sample size	Fieldwork
YouGov	52	20	19	6	3	Online	1144	2-4.5
Savanta ComRes	42	25	22	8	3	Online	1001	30.4-4.5
Survation	49	21	21	8	2	Online	1008	30.4-4.5
Ipsos MORI	50	20	22	6	3	Telephone	1502	30.4-3.5
Opinium	51	23	19	7	1	Online	1015	28.4-3.5
Average	48.8	21.8	20.6	7.0	2.4
Result	47.7	21.9	21.6	6.9	1.9
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

	SNP	Con	Lab	Lib Dem	Green	Other
YouGov	38	22	16	5	13	6
Savanta ComRes	34	23	19	6	9	8
Survation	36	21	19	7	10	6
Ipsos MORI	39	23	18	4	12	4
Opinium	41	23	17	6	8	5
Average	37.6	22.4	17.8	5.6	10.4	5.8
Result	40.3	23.5	18.0	5.1	8.1	5.0
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

	SNP	Con	Lab	Lib Dem	Other	Method	Sample size	Fieldwork
Panelbase	48	21	20	7	4	Online	1096	28-30.4
BMG	49	19	21	9	3	Online	1023	27-30.4

List Vote

	SNP	Con	Lab	Lib Dem	Green	Other
Panelbase	39	22	16	7	8	8
BMG	37	22	17	8	9	9

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 Cymru	Con	Lab	Lib Dem	Other	Method	Sample size	Fieldwork
YouGov	20	29	36	3	11	Online	1144	2-4.5
Savanta ComRes	18	28	36	6	12	Online	1002	29.4-4.5
Average	19.0	28.5	36	4.5	11.5
Result	20.3	28.1	39.9	4.9	6.8
Difference	-1.3	+0.4	-3.9	-0.4	+4.7

List Vote

	Plaid Cymru	Con	Lab	Lib Dem	Green	Abolish	Other
YouGov	21	25	31	4	5	7	6
Savanta ComRes	19	25	32	5	5	6	9
Average	20.0	25.0	31.5	4.5	5.0	6.5	7.5
Result	20.7	25.1	36.2	4.3	4.4	3.7	5.6
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.

	Con	Lab	Lib Dem	Green	Other	Method	Sample size	Fieldwork
Panelbase	29	40	8	6	14	Online	1002	4-5.5
YouGov	31	43	5	10	11	Online	1144	2-4.5
Savanta ComRes	29	41	8	5	17	Online	1001	29.4-4.5
Opinium	29	48	8	7	8	Online	1005	28.4-3.5
Average	29.5	43.0	7.3	7.0	12.3
Result	35.3	40.0	4.4	7.8	12.5
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.