Second combined forecast for the 2024 general election

By Stephen Fisher, John Kenny, Paul Furey, and Polina Ryzhuk. 6^th June 2024.

All the different sources of seat forecasts still point to a large Labour majority. On average across the different kinds of forecast, the predicted majority is 197. That is up slightly from the 188 average majority in our first combined forecast this time last week.

Table 1. Seat Forecasts

	Betting Markets	Complex models	Simple models	MRP	Average
Con	118	144	195	114	143
Lab	424	434	392	444	424
LD	42	32	28	45	37
Reform	3	0	0	1	1
Green		1	1	1	1
SNP	19	16	13	24	18
PC		3	3	3	3
Lab majority	199	217	134	238	197

The increase is largely due to the higher Labour forecasts from the MRP models, which are listed in Table 2. They are remarkably different from each other, ranging from a Labour majority of 114 to as much as 324. The forecast number of Conservative seats in the MRP projections range from 66 to 180. A key part of the differences between the MRP projections for the Conservatives and Labour is the degree to which the models expect party performance to be proportional to the prior Conservative vote share. By comparison, traditional uniform change models (under simple models in Table 1) produce more modest forecasts for Labour gains and Conservative losses.

Table 2. Recent MRP seat forecasts

	Lab	Con	LD	Reform	Greens	SNP	PC	Majority
Electoral Calculus	485	66	59	0	2	17	3	320
More in Common	382	180	30	0	1	35	3	114
YouGov	422	140	48	0	2	17	2	194
Survation	487	71	43	3	0	26	2	324
Average	444	114	45	1	1	24	3	238

Table 3 shows estimates of probabilities and pseudo probabilities for different election outcomes. Averaging the different sources suggests a 95% chance of a Labour government and an 87% chance of a Labour majority, with little change since last week.

Table 3. Probabilities (%)

	Betting markets	Models	Polls	Citizen forecast	Volunteered	Average
Lab Majority	95	96	100	58		87
Lab 100+ Majority			71			71
Con led Gov	6		0	12	0	5
Lab led Gov	96		100	85	100	95

Table 4 summarises forecasts for vote shares. Again, very little change from last week. On average, Labour are forecast to have a 21-point lead over the Conservatives, up by 1 point from last week.

Table 4. Vote share forecasts (%GB except where stated)

	Poll aggregates	Betting markets	Models	MRP	Average
Con	24	23	23	25	23
Lab	45	43	45	43	44
Lib Dem	9	12	9	10	10
Reform	11	14	12	10	12
Green	5		6	5	6
SNP (in Scotland)	31		31		31
PC (in Wales)	12		11		12
Lab-Con lead	21	20	22	18	21

Methodology

The idea of combining forecasts from different sources has a good track record, though it has to be admitted that our attempts for the 2017 general election and the 2016 Brexit referendum did not work out so well. For reviews of previous combined forecasts here for 2017 and here for 2019. Also worth noting is the experience of the pollyvote.com combined forecast of the US presidential elections.

The basic approach is to combine forecasts by averaging them within each category and then average across categories. Since the different sources do not all present similar figures that can be averaged on a like-for-like basis there are various judgement calls we have had to make on how to treat the data.

Polls

For vote shares, we use the various available polling averages. Some admittedly are quite sophisticated, allowing for house effects, but they are nonetheless estimates of current public opinion and not future votes.

To estimate a pseudo-probability of a Labour majority from the polls, we take the proportion of pollsters with a GB Labour lead greater than 12 points, on the basis that this is the level required for a Labour majority under traditional uniform change assumptions. Similarly, the proportion of the most recent polls from each pollster with a lead of 19 points or more serves as a pseudo probability for the chances of a Labour majority of over 100 (making allowances for current polling in Scotland and Wales).

Statistical models

We have divided statistical models into simple (poll average plus uniform swing seats projections), Multilevel Regression and Poststratification (MRP), and other complex models (anything more elaborate than uniform change but not necessarily particularly complex). Within these categories we simply average the available estimates of seats and shares. We have not excluded any models based on quality, but they do have to be statistical models as opposed to personal guesses.

Betting markets

For the seats forecasts we are using the mid points of the spread betting for the markets. The markets might collectively imply fewer or more seats forecast for the parties in total than there are seats to be won in Britain. This is because the markets are separate for each party and do not need to be consistent collectively.

Betting markets for shares of the vote are typically done in bands. We take the mid points of the bands with the highest implied probabilities weighted by their implied probabilities from the betting odds. We then average across bookies.

Citizen forecasts

Some polls ask people what they think that the outcome of the election will be. We call these ‘citizen forecasts.’ Different pollsters use different survey questions but they can be combined to generate pseudo-probabilities. We use the proportion of poll respondents who think there will be a Labour majority, excluding “don’t knows” and re-percentaging, as the pseudo-probability of a Labour majority. We treat the question of whether Rishi Sunak will continue as prime minister to be equivalent to the Conservatives getting a majority, because they do not have any clear governing partners in the event of a hung parliament. Where appropriate, the probability of a Labour led government is taken to be 100 minus the probability of a Conservative led one. But that is not the case for questions where respondents are given the chance to say they think there will be some “other” kind of government. We use the most recent poll with a citizen forecast survey question from each pollster and only those in recent weeks. Only a few pollsters ask citizen forecast questions.

Volunteered forecasts

These come from the Good Judgement Project, which encourages people to forecast the outcomes of various events in order to develop and improve their forecasting skills. The people providing their predictions to the Good Judgement Project are not necessarily experts, but nor can they be seen as a representative sample, and so we report their predictions separately as ‘volunteered’ forecasts.

Estimates come from late on 5^th June 2024.

Sources:

Betting Markets:

Models (Elaborate):