All posts by electionsetc

Forecasts for the US Midterm Elections 2018

By Stephen Fisher.

The most striking thing about the forecasts for today’s midterm elections in the United States is that they have been much less talked of in the media than in previous campaigns. This is partly because in 2016 most of the forecasters put very high probabilities (90%+) on Hilary Clinton winning the presidency. (See here for a post-mortem.)

This post reviews the main statistical model based forecasts for the US House and Senate, with some discussion of the methodology and comparison with other forecasts. Overall, and as usual, there is not much variation between the forecasters in their central forecasts. They all point to the Democrats taking control of the house and the Republican retaining control of the Senate. The striking exception is a Gallup poll suggesting 50% think the Republicans will retain control of the House and only 44% think the Democrats will win it.

Despite the forecasts differing from the expectations of the American people, the forecasts appear to have been widely accepted in the media. So much so that some journalists suggest it will be a vindication of Donald Trump if Republicans maintain control of the House. However, if that happens it will most likely be despite a clear lead for the Democrats in the popular vote. In which case, it would be the electoral system, not Trump, that thwarts the Democrats. Meanwhile, if there are net Republican gains in the Senate it will be primarily because the Democrats are defending a big haul from 2012.

House forecasts

As James Campbell has noted, in all but three midterm elections since 1900 the President’s party has lost seats. Since 1950 the average loss has been 24 seats. The Democrats need to make net gains of 23 or more to take control.

Continue reading Forecasts for the US Midterm Elections 2018


How big a lead does Labour need in the local elections to be on course to win the next general election?

By Stephen Fisher.

People often wonder what local election results bode for the next general election. Here is a quick look at the relationship between the BBC Projected National Share of the vote (PNS) in the local elections and the outcome of subsequent general elections. In particular, I focus on the lead for the main opposition party over the principal governing party. The graphs below show the lead at the subsequent general election plotted against the lead in the PNS.

Continue reading How big a lead does Labour need in the local elections to be on course to win the next general election?

Calculating the local elections Projected National Share (PNS) in 2018

By John Curtice and Stephen Fisher.

Part of the ritual of election-night coverage at the BBC is the calculation of the so-called Projected National Share (PNS). This is an estimate of the share of the vote that the principal parties would have won in a GB-wide general election if voters across the country as a whole had behaved in the same way as those who actually voted in the local elections. It provides a single, seemingly straightforward measure of party performance that can tell us not only how well or badly a party has done as compared with four years ago, but also as compared with any previous local elections for which a PNS is available – even though the places in which local elections are held varies considerably from one year to the next.

One of the key patterns in last year’s general election results was a tendency for those who voted Remain to swing more to Labour than those who voted Leave, while the Conservatives lost ground amongst Remain voters while advancing amongst their Leave counterparts. If a similar pattern is maintained at these local elections – and it was in last year’s county council elections – then the Labour vote will increase more (or fall less) were the Remain vote was higher in 2016, while the converse will be true of the Conservatives.

Continue reading Calculating the local elections Projected National Share (PNS) in 2018

Forecasting Local Election Seat Gains/Losses 2018

by Stephen Fisher

My forecasting model for seat gains/losses at local elections has previously been a simple model based on change in party support in the polls. While the historical data show that changes in the percentage of the council seats that a party wins is reasonably strongly correlated with changes in that party’s poll share, that basis for forecasting this year would not work.

The Conservatives are up by 8 points and Labour up by 6 points in the polls since just before the 2014 local elections, when most the seats up for election this week were last fought. As in last year’s general election, support for both main parties has increased largely because of a big drop in UKIP support. Forecasting the parties separately based on changes in their respective poll shares would produce misleading predictions of big gains for both main parties. In first-past-the-post elections it is typically relative not absolute performance that matters for seat outcomes. Last year, despite winning more votes the Conservatives lost their parliamentary majority because the Labour vote share went up by more than did that for the Conservatives.

For this reason, my forecasting models this year are based on changes in the gaps between polls shares.  For the Conservatives, who have traditionally faced many contests with the Liberal Democrats, their leads over both Labour and the Liberal Democrats matter. For Labour, the model is primarily based on the Labour lead over the Conservatives. Meanwhile, for the Liberal Democrats, their changing opinion poll performance relative to the Conservatives, but not Labour, has historically been correlated with headline local election seat changes.

Continue reading Forecasting Local Election Seat Gains/Losses 2018

How Theresa May lost her majority and how Scots have kept the Tories in government

By Stephen Fisher.

At the time of writing it looks like the Conservatives will win 319 seats, short of the 326 seats they needed to win an overall majority. (The following numbers are provisional.)

Most likely they will come to some agreement with the Democratic Unionist Party in order to remain in government.

Given that Sinn Fein won 7 seats that they will not take, there are effectively 643 MPs returning to the House of Commons.  The Conservatives 319 + 10 DUP seats constitutes an effective majority of 15.

This is intriguingly similar to the actual majority of 12 that David Cameron won in 2015.

By comparison with a tally of 331 seats in 2015, the Conservatives lost 27 seats to Labour and 5 to the Liberal Democrats.

29 of those 32 Conservative losses were in England.

They were partly compensated for by gains of 6 seats from Labour, 1 from the Liberal Democrats, 1 from UKIP in England.  As a result the Tories suffered a net loss of 21 seats in England.

They lost a further 3 seats in Wales, making a net loss of 24 seats in England and Wales.

Were that the end of the story then the Conservatives would have ended up with just 310 seats. That would not have been enough to remain in government.

Only thanks to 12 gains from Scottish National Party are the Tories able to remain in government.

The Conservatives in England and Wales owe their continuing position in government to the popularity and success of Ruth Davidson and the Scottish Conservatives.

Numbers will be updated when all results are in.




Final Combined Forecast for GE2017

By Stephen Fisher, John Kenny and Rosalind Shorrocks.

Our final combined forecast is for a Conservative majority 66. This estimate is down only slightly from 70 on Friday last week, but well down on the 132 majority we first forecast on 12th May. All three sources on average each suggest that net gains for the Conservatives will largely be at the expense of Labour and the SNP, with tallies for other parties little changed from 2015.

Seats Betting Markets Complex models Simple models Average
Con 371 361 343 358
Lab 199 214 228 214
LD 11 7 7 8
UKIP 0 0 0 0
Green 1 1 1 1
SNP 46 46 50 47
PC 3 3 3 3
Con majority 92 72 36 66

The combined probability of a Conservative majority is now 72%. This is little changed from last week but down from 91% in our first combined forecast. The probability of a 100+ majority Conservative landslide is, at 11%, the same as last week but well down on the 71% we reported on 12th May. Betting markets are the most confident of a Tory majority. Citizens collectively express the most uncertainty, with both the highest probability on a Tory landslide and the second highest probability on the Conservatives losing their majority. Taking the most recent poll from each pollster, only just over half of them show the Conservatives with the lead of 6 points or more that would be needed for the party to win a majority on uniform change calculations. The 54% figure is down from 63% last week, but it reflects a change in who has done a poll this week more than how the pollsters from last week have seen their figures change.

Betting markets Models Polls Citizen forecast Average
Conservative Majority 0.83 0.82 0.54 0.69 0.72
Conservative landslide 0.17 0.00 0.17 0.11

For our final forecast we have taken a straight average of the final polls published at the time of writing instead of relying on other poll aggregators. This average is for a strikingly similar Conservative lead as in the actual outcome last time 2015, 6.5 compared with 6.6 points. But the final polls last time pointed to a tie. The markets and models predict that the polls are over estimating Labour and under estimating the Conservatives again. Overall our combined forecast this week now suggests that the Conservatives will win with a 9.6 point lead. This estimate is down only slightly from 10.2 points last week. But there has been a massive drop from the 18.7 point lead predicted in our first combined forecast on 12th May.

Final Poll average Betting markets Models Average
Con 43.0 45.8 44.3 44.4
Lab 36.5 34.7 33.1 34.8
Lib Dem 7.6 8.3 9.7 8.5
UKIP 4.4 3.4 4.7 4.1
Green 2.1 2.2 2.1
SNP 4.0 4.1 4.0
PC 0.6 0.8 0.7
Con-Lab lead 6.5 11.1 11.2 9.6


The main changes in method this week are as follows.

Nigel Marriott in his final model now includes a forecast for party vote shares as well as the previous seat forecasts and so this is included; we have also added new models from Election Data, Elections Etc, Janta-Lipinski, Kantar Public, Michael Thrasher, Andreas Murr et al and Number Cruncher Politics. YouGov’s forecast model has been removed because they have officially dropped it. Betting market data for the forecasted party vote share of Labour, the Liberal Democrats and UKIP is gathered from Betfair/Betfair Exchange where such data is not available on Oddschecker.

Our polling average is now a simple average of the final polls.

As there have been five citizen surveys published since the 31st May, we have decided to limit the inclusion criteria for this so that only the most recent citizen survey from each survey company during this period is included.

Overall description of the method

Our basic approach is to combine forecasts by averaging them within each category and then taking the average across categories. Since the different sources do not all present equally clear figures that can be averaged on a like for like basis we have made various judgement calls on how to treat the data.

Historically the idea of combining forecasts from different sources has had a good track record, though it has to be admitted that our attempt to do one for the EU referendum did not work out well. Most recently the combined forecast of the US presidential election last year was 2 points out on the share of the vote. 


Our polling average is now a simple average of the final polls. Previously for vote shares, we use the various available polling averages, or ‘polls of polls’, and take their average. We exclude polling averages for whom the most recently published polling average is more than a week old. There are a number of different polling averages. They are in truth nowcasts rather than forecasts, but we are in effect treating them as forecasts. Some admittedly are quite sophisticated, allowing for pollster (aka house) effects, but they are nonetheless estimates of current public opinion and not future votes.

We do not attempt to say what seats outcome is implied by polls (that is the job of the modellers). However, since statistical models are rarely if ever clear about the probabilities their models place on key events like a Conservative majority and a 100+ majority, we have included in the probabilities table some pseudo-probabilities from the polls. Taking the most recent poll published by each pollster in the last week, we calculate the proportion showing a Conservative lead over Labour of more than 6 points as the pseudo-probability of a Conservative majority, but we allow polls with exactly a 6 point lead to contribute a 0.5 to the average. Using the same polls, we use the proportion showing a Conservative lead over Labour of 16 points or more as the pseudo-probability of a Conservative landslide. These thresholds of 6 and 16 points are based on what would be required under uniform swing assumptions for the Conservatives to win a bare majority and a 100+ majority respectively.

Betting markets

There are numerous betting markets for the various outcomes in the election. We have taken those that are most helpful for the four forecasts we want to produce. For seat shares, we take the mid-point of the spread as the seat share, and average these mid-points between different sources. Note that the markets imply fewer seats forecast than there are actually are in the House of Commons. This is because the markets are separate for each party and do not need to be consistent collectively.

For vote share, we use betting markets for the Conservatives, Labour, the Liberal Democrats and UKIP. (Vote share markets for other parties are unavailable either on the betting market aggregation site Oddschecker of Betfair/Betfair Exchange). Odds are given for 5-point ranges of vote share. We take a weighted sum of the mid-points of these ranges where the weights are the implied probabilities. For the top and bottom options we use 2.5% above/below the upper/lower bound (e.g. 52.5 for “Above 50” and 27.5 for “Below 30”). The weighted sum is calculated just using the three categories with the largest implied probabilities, because the probabilities for other categories are so small and unstable. For UKIP we use just the two most likely categories.

For the probability of a Conservative majority we give an average of the implied probability from sites offering this market. For the probability of a Conservative landslide, we use the combined prices PredictIt that the Conservatives will win 370-379 seats, 380-389 seats, and 390 or more seats. This really represents a majority of 90 or more but that was as close as we could get to 100.

Statistical models

There are numerous statistical forecasting models this year. We have divided them into two categories: simple (poll average plus uniform swing seats projection) and complex (anything more elaborate than the simple models, although they are not necessarily particularly complex). Some make adjustments for long run differences between pollsters, for constituency variation, and some estimate by how much things will differ between current polls and the eventual result. Chris Hanretty’s forecast at does all of these things. Within these categories we simply average the available estimates of seats and shares. In addition, as discussed above, we have separated those models based on the local election results into a separate category.

We should note that not all of the models are the modellers’ favourite. Some are counterpoints to their main models for comparison. We have included these on the basis that they are still talked of and expected to be reasonable estimates, for example, at Electoral Calculus, Martin Baxter has a local election results based model. We have not excluded any models based on our judgement of quality, but they do have to be statistical models as opposed to personal guesses.

Citizen forecasts

Some polls ask people what they think that the outcome will be on June 8th. Different pollsters use different survey questions but they can be combined to generate pseudo-probabilities. We use the proportion of poll respondents who think the Conservatives will win/there will be a Conservative majority, excluding don’t knows and re-percentaging, as the pseudo-probability of a Conservative majority. We similarly use the proportion of poll respondents who think that there will be a Conservative landslide/the Conservatives will win more than 100 seats, for the probability of a Conservative landslide.

Note: Estimates come from around 9am on the morning of  8th June 2017. In all seat estimates, the Speaker’s seat is counted as Conservative.


There sources we used are listed below in no particular order. Please let us know of any that you think we have missed or misclassified. Some polling averages we know of were not included because they were more than a week old.

Prediction markets:
Sporting index
Betfair Predicts


Betfair Exchange


Complex Forecasting Models:
Chris Prosser (Chris Hanretty)
Electoral Calculus (main and local election forecast)
Election Data

Elections Etc

Forecast UK 

Kantar Public

Lord Ashcroft

Michael Thrasher
Murr, Stegmaier and Lewis-Beck

Nigel Marriott

Number Cruncher Politics

PME Politics (Patrick English)



Simple forecasting models (polling average + uniform swing):


Polling Averages (less than a week old):
Britain Elects
The CrossTab


Citizen forecasts:
YouGov (here)
ComRes (here)
ICM (here)

Opinium (here)

Survation (here)

The three authors are equal contributors and our names are in alphabetical order.


Polls-based forecast for the 2017 British general election

by Stephen Fisher and Josh Goldenberg.

Our central forecast for the result of today’s general election is as follows.

  GB share of the vote Seats 90% prediction intervals
Conservative 44 349 318-385
Labour 34.5 223 192-252
Liberal Democrat 9 9 3-15
SNP 4 47 39-53
PC 1 3  
UKIP 4 0  
Green 2.5 1  

This implies a Conservative vote share lead over Labour of 9.5 points and a majority of 48.  This represents only a modest improvement over the party’s performance 2015 when they achieved a majority of 12 with a 6.6 point lead.

From this central forecast, our estimated probability of a Conservative majority is 87%. Our analysis gives just a 1% chance of the Conservatives winning a 100+ landslide majority.

Continue reading Polls-based forecast for the 2017 British general election