Fourth combined forecast for the 2024 general election

By Stephen Fisher, John Kenny, Paul Furey, and Polina Ryzhuk. 19^th June 2024.

All the different sources still point to a large Labour majority. On average across the different kinds of forecast, the predicted majority is, at 200, up eight from last week. The combined forecast for the Conservatives is down by nine seats, and that for the Liberal Democrats is up by seven seats since last week. Those changes have been largely driven by changes in MRP projections, but other sources have also typically changed in the same direction.

Table 1. Seat Forecasts

	Betting Markets	Complex models	Simple models	MRP	Average
Con	90	132	187	97	127
Lab	435	425	389	453	425
LD	52	49	35	54	48
Reform	4	2	0	3	2
Green	2	1	1	2	1
SNP	22	18	17	20	19
PC	4	3	3	3	3
Lab majority	220	200	128	256	200

Table 2 below lists the latest MRP estimates. Ipsos and Savanta published their first MRP projection this week. There have been updates from the other four within the last week (Electoral Calculus, More in Common, Survation, and YouGov). All of those four revised their Liberal Democrat seat projections up. Those MRP models that had higher (but still low) figures for the Conservatives (More in Common and YouGov) have come down, with little change from Electoral Calculus and Survation. Ipsos estimates for the Conservatives and Labour were close to the average of the first four to publish, but lower for the Liberal Democrats. Savanta has the highest forecast for Labour (516) and lowest for the Conservatives (53).

Table 2. MRP seat forecasts

	Lab	Con	LD	Reform	Greens	SNP	PC
Electoral Calculus	461	80	63	1	2	20	3
Ipsos	453	115	38	3	3	15	4
More in Common	406	155	49	0	1	18	2
Savanta	516	53	50	0	0	8	4
Survation	456	72	56	7	1	37	2
YouGov	425	108	67	5	2	20	4
Average	453	97	54	3	2	20	3

Figure 1 shows how the forecasts of the size of a Labour majority have changed since the start of the campaign. The betting markets seem to be increasingly convinced by the MRP forecasts, but the very latest MRP estimates came through after the betting market figures were compiled this week.

Figure 1. Forecasts for the size of a Labour majority

Table 3 shows estimates of probabilities and pseudo probabilities for different election outcomes. Averaging the different sources suggests a 94% chance of a Labour government and an 89% chance of a Labour majority. Practically no change since last week. Sadly, yet again no new citizen forecast polls that we could find this week. The figures for citizen forecasts in Table 3 are now based on just two polls (from Redfield and Wilton and YouGov) both from 2^nd June, which is a shame.

Table 3. Probabilities (%)

	Betting markets	Models	Polls	Citizen forecast	Volunteered	Average
Lab Majority	96	93	100	68		89
Lab 100+ Majority			71			71
Con led Gov	4		0	18	0	6
Lab led Gov	97		100	80	100	94

Figure 2 shows how stable the predicted probabilities of a Labour majority have been since the start of the campaign.

Figure 2. Predicted probabilities of a Labour majority

Table 4 summarises forecasts for vote shares. The Conservatives and Labour are down a point or two in the polls and models, but with little change in the MRP and betting market averages. The net effect on the overall average is Labour (and their lead over the Conservatives) are down one point. So, on average, Labour are forecast to have a 20-point lead over the Conservatives. Reform UK, are up a further point, to 15 per cent. No net change since last week for the Liberal Democrats and the Greens.

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

	Poll aggregates	Betting markets	Models	MRP	Average
Con	21	21	21	24	22
Lab	42	42	42	43	42
Lib Dem	11	12	11	11	11
Reform	15	18	15	13	15
Green	6		6	5	6
SNP (Scotland)	30		31		31
PC (Wales only)	15		13		14
Lab-Con lead	21	20	21	19	20

Figure 3 shows the forecasts for the Labour lead over the Conservatives in the GB share of the vote. The forecasts from the different sources have been remarkably stable and similar to each other.

Figure 3. Forecasts for the Labour lead in vote share

Estimates come from data collected late afternoon and early evening on 19^th June 2024.

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.

Sources:

Betting Markets:

Models (Elaborate):

Bradshaw Advisory