First combined forecast for the 2019 general election

By Stephen Fisher, John Kenny and Rosalind Shorrocks (Universities of Oxford, Southampton and Manchester respectively)

There aren’t so many different forecasts for the general election out yet, but enough to start looking at how they compare. This is the first of hopefully weekly blogs where we review the different forecasts from different methods and combine them into an overall forecast. At the moment, the polls and seats forecasts suggest a comfortable Conservative majority but citizens and betting markets are not so sure it will happen.

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 well. We will write more about those experiences soon. Also worth noting is the experience of the pollyvote.com combined forecast of the US presidential elections.

Here we aggregate seats and vote share forecasts from a variety of sources including betting markets, polls, statistical forecasting models and citizen forecasts. As well as updating weekly, the methodology (as detailed below) might well evolve. So comments and suggestions on our approach and for new forecasts to include are welcome.

All the different sources point to the Conservatives being comfortably the largest party, but with some variation in how well the party will do. On average across betting markets, complex and simple models the Tories are expected to win with a comfortable majority of 57. Labour are projected to be down heavily with gains for the Liberal Democrats and Scottish National Party (SNP). Betting markets are rather cautious compared with the forecasting models.

Seats Betting Markets Complex models Simple models Average
Con 338 364 358 353
Lab 212 185 189 195
LD 43 43 29 38
Brexit 3 0 0 1
Green 2 1 1 1
SNP 50 50 50 50
PC 5 4 4 4
Con majority 27 78 66 57

The forecasts of the overall outcome so far are all heavily influenced by the headline shares of the vote from published opinion polls. All of them point to the Conservatives winning a majority on standard uniform change calculations, and one of the eight most recent polls from each pollster since the campaign started has a lead of 16 points which suggests a majority of over 100. That information is the basis for the pseudo probabilities in our table below, a crude methodology we may change. By contrast the betting markets, Good Judgement Project volunteered and particularly the citizen forecasts are much more circumspect. How the betting markets for seats and the betting market for a Conservative majority can be reconciled, we’re not sure. Overall the probability of a Tory majority is just 51%.

Betting markets Polls Volunteered Citizen forecast Average
Conservative Majority 0.48 1.00 0.36 0.22 0.51
Conservative Landslide 0.25
Boris Johnson as next Prime Minister 0.74 0.25 0.47 0.49

The sources for vote-share forecasts are remarkably consistent across polls, markets and forecasting models, with the betting markets again somewhat more cautious than the polls about the prospects for a big Conservative lead.

% vote share Polling averages Betting markets Models Average
Con 37.8 38.1 38.4 38.1
Lab 25.7 27.4 27.2 26.8
LD 16.9 18.7 15.7 16.3
Brexit 10.2 6.7 9.6 8.8
Green 3.7 3.0 3.3
SNP 3.8 3.5 3.6
PC 0.8 1.7 1.3
Con-Lab lead 12.1 10.6 11.2 11.3

Methodology

The basic approach is to combine forecasts by averaging them within each category and then averaging across categories. Since the different sources do not all present similar clear 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.

There are numerous statistical forecasting models this year (and more to come). We have divided them between simple (poll average plus uniform swing seats projection) and complex (anything more elaborate 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.

For the seats forecasts we are using just the mid points of the spread betting for the markets. Note that the markets might imply fewer or more seats forecast for the main parties than there are 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 up to a total probability of 100%, averaging across bookies.

We do the same for seat forecasts where figures are provided in bands. When spread betting is instead provided, we use the mid points of the spread betting for the markets. Note that the markets might imply fewer or more seats forecast for the main parties than there are in Britain. This is because the markets are separate for each party and do not need to be consistent collectively.

Estimates come from around lunchtime on 6th November 2019.

Sources 

Citizen forecasts:

Deltapoll (here)

Opinium (here)

Survation (here) 

Forecasting models:
Electoral Calculus

Forecast UK

UKelect

Principalfish

Polling Averages (less than a week old):
BBC

Britain Elects
ElectionForecast

FT
Principalfish
UKElect

 

Prediction markets:
Betfair

Betfair Exchange

Betfred

Ladbrokes

Oddschecker

Paddy Power

Predictit
Smarkets

Sporting index

Volunteered:

Good Judgment Project (here)

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