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UK General Election 2024 #3 – My Forecast as of 23rd June 2024

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My first forecast of the number of seats to be won by each party in the 2024 UK General Election is –

  • CON 108
  • LAB 442
  • OTH 100

My Articles on the 2024 UK General Election
  1. The most accurate forecaster of the 2019 general election – I was independently assessed as the most accurate forecaster beating even Sir John Curtice’s exit poll.
  2. Keir Starmer’s Train to Downing Street – My assessment in 2021 of what Labour needed to do to win the next election.
  3. My election forecasting Track Record 2010 to 2024 – A list of all election forecasts I have made for General, European and Local elections, how I made them, how they turned out and what lessons I learned.
  4. How Accurate are Voting Intention Polls (Revised) – A recent article which explains why I now think the polls are accurate when before they would often underestimate the Conservative lead over Labour.
  5. Going Beyond the Swing in 2024 – A preliminary look at the 2024 general election at the start of the year.  I give probabilities for 10 specific outcomes.
  6. My 2024 UK General Election Forecasting Model – I use a top down forecasting approach based on the sum and difference in Conservative and Labour vote share to predict share of seats won.

 

Data used in this article

All polling data prior to April 2024 comes from Mark Pack’s invaluable Pollbase.  For data since April, I am using the BBC poll tracker.  For a summary of what the polls are saying at the time of this article, see this X/Twitter thread.

All electoral data I display and use in this article comes from the House of Commons Research Library.  The PDF file I use most of the time is this one UK Election Trends 1918-2019.  I’ve created a spreadsheet GB General Election Data 1918-2019 – votes and seats which contains all the data used to build my models.  For clarity –

  • Seat Share is the percentage of seats in Great Britain won by each party.
  • National Vote Share is the percentage of all votes cast in Great Britain for each party.
  • Average Vote Share (per Seat) is the average of the vote share in each British constituency the party stands in.

I first explained the national and average vote share concepts in my article “How accurate are voting intention polls? – Revised“.

 

Reminder of my forecasting model for 2024

I am using a top down model to predict total number of seats won by the major parties based on the sum and difference in the Conservative and Labour party average vote share per seat.

Once I’ve estimated sum seat share CON+LAB and seat share difference CON–LAB, I will have two simultaneous equations which can be solved to estimate the percentage of the 632 seats in Great Britain that will be won by the Conservative and Labour parties.  I explain my model in full here.  For now, my model does not yet split seats won by other parties into the separate parties e.g. Lib Dems, SNP, etc.  I am working on this and will include this feature when ready.

One of the questions I have to resolve is which of the fitted lines in the above charts should I use.  In fact I will be using simulation where each line has a certain probability of being chosen.

For CON+LAB, the probabilities I’ve decided to use are –

  • Black line 5% – it’s been over 30 years since this was the fit hence the low probability
  • Green line 75% – the last 7 elections have used this fit hence why I say it is the most likely
  • Brown line 20% – given the high probability of a new low for CON+LAB vote share, I am not convinced the green line will hold hence why I’ve given this a 1 in 5 chance of this occurring.

For CON–LAB, the probabilities I’ve decided to use are –

  • Black line 25% – the last 3 elections have sat on this fit so it can’t be ruled out.
  • Green line 75% – the last 3 times where Labour kicked out a Conservative government with a big swing (1929, 1945, 1997) all sat on the green line.  Polls are pointing to something similar in 2024.

The most likely outcome is that both models use their green fitted lines.  This is what I will use to illustrate my forecast.

 

Reminder of how accurate the polls are

I recently realised voting intention polls on average do predict the vote share difference CON–LAB provided one compares polls with average vote share per seat rather than national vote share.

When it comes to the sum vote share CON+LAB, polls are accurate on average over the whole timescale 1950 to 2019 but that hides notable differences over time.  The last four elections, coinciding with a rise in number of pollsters and more web polling, underestimated the total vote share of the two main parties by 3% on average.  The step change in 2010 is statistically significant.

 

What do the latest polls say?

For the week ending 21st June, using published data from 16 pollsters, Labour has a 20 point lead over the Conservatives.  This would be a new record for Labour if this occurs on 4th July.

At the same time, the combined CON+LAB vote share of 61% would be the lowest on record since the CON/LAB duopoly began in 1922, beating the 66.6% in 2010.  More than that, the current 7 day trend in the CON+LAB vote share is -1.4% per week.  if extrapolated to 4th July, we could be looking at 59% instead of 61%.

Reform are in 3rd place and are still trending upwards.  Some polls already have Reform ahead of the Conservatives.  If this were to happen by the eve of the election, I expect to dump the model I am using here in favour of a variant on the forecasting model I used in 2015.

 

How is a forecast made?

My official forecast is an average of 1000 simulations of possible scenarios.  The process works like this.

My first simulation assumes the polls are correct and that both the sum seat share and seat share difference models use the green fit lines.  I call this the Illustrative Example and the calculation works like this.

  • The estimated sum seat share CON+LAB is 87.7% using the green fit line i.e. Other parties will win 12.3% of the 632 seats in Great Britain.
  • The estimated seat share difference CON–LAB is -55.0% using the green fit line.
  • The estimated CON seat share is 16.3% = (sum+diff)/2 = ( 87.7% + -55.0% )/2
  • The estimated LAB seat share is 71.3% = (sum-diff)/2 = ( 87.7% – -55.0% )/2

With 632 seats in all of Great Britain, the illustrative example results in 103 CON, 451 LAB and 78 OTH seats.

Thereafter, the other 999 simulations are processed as follows –

  1. A random number is added to the CON+LAB vote share indicated by the polls.  Given the notable step change in 2010, I am currently drawing a random number from a normal distribution with mean +2.0% and standard deviation 1.0%.  I am keeping this element under review.
  2. A random number between 0 and 1 is created and this decides whether the fitted model for sum seat share CON+LAB uses the black (5%), green (75%) or brown(20%) fitted lines.
  3. The chosen fitted line is used to estimate the sum seat share CON+LAB.
  4. To the estimated sum seat share CON+LAB, a random number drawn from a normal distribution with mean 0% and standard deviation 0.5% is added to allow for residual error.
  5. A random number is added to the CON–LAB vote share indicated by the polls.  This is more straightforward as it is a random draw from a normal distribution with mean 0.0% and standard deviation of 2.5%.
  6. A random number between 0 and 1 is created and this decides whether the fitted model for seat share difference CON–LAB uses the black (25%) or green (75%) fitted lines.
  7. The chosen fitted line is used to estimate the seat share difference CON–LAB.
  8. To the estimated seat share difference CON–LAB, a random number drawn from a normal distribution with mean 0% and standard deviation 2.5% is added to allow for residual error.

On completion of all simulations, the average of the estimated sum seat share CON+LAB after step 4 and the estimated seat share difference CON–LAB after step 8 are calculated.  These are then solved to get the estimated number of seats to be won by the Conservatives, Labour and Others.  The 1,000 simulations provide data on possible margins of error.

As of 23rd June 2024, my simulations gave me my first official forecast as follows –

  • CON 108 seats (88 to 127)
  • LAB 442 seats (459 to 429)
  • OTH 82 seats
  • Northern Ireland parties 18 seats

The ranges in brackets represent the interquartile range of my 1000 scenarios i.e. from the 25th to the 75th percentile.  Note the error in CON and LAB are inversely correlated hence why Labour’s range is from high to low i.e. if the Conservatives do end up with 88 seats, Labour are more likely to get around 459 seats.

 

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