BRIEFING NOTE: General Election 2015 Polling

REUTERS/Peter Nicholls

REUTERS/Peter Nicholls

With just one day to go before the British public cast their vote at the ballot box, the ground war is hotting up. James Downes examines the current state of the polls in looking at both vote and seat shares, before outlining the importance of campaigning for the Conservative Party on the final day of the election.

The State of the Polls

The last few years in UK polling have seen more sophisticated techniques being used to survey voters, with market research companies such as YouGov leading the way in online methodology. The last few weeks have also highlighted the variations in the methodology that pollster’s use, with some political parties advancing their claim based on a few percentage point differences that are likely to be the result of the different methodologies used (telephone, face-to-face, and internet) alongside margins of error that is factored into every poll.

Much has also been made of the election forecasting model used by the American statistician Nate Silver. Silver’s model correctly predicted the winner of all 50 states and the District of Columbia in the race for the 2012 United States Presidential election. In the same year, Silver’s model was also applied to U.S. Senate races, correctly predicting 31 out of 33 seats. Whilst the Bayesian statistics and number crunching that Silver deploys are complex, the model’s power to predict political phenomena are impressive. Researchers in fields such as Economics and Political Science know how difficult it is to accurately predict human behaviour and Silver’s model seems to buck this trend.

Election Models

When turning to current election forecasting models, compelling statistical coverage and analysis is provided by Election Forecast UK.  The model that Election Forecast UK uses combines data provided by YouGov with all publicly released national and constituency polls, alongside historical election results, and data from the UK Census.

Current forecasts as of the 5th May show the Conservatives on 34.5% and Labour on 32.2%. This is provided in Table 1 below. The Liberal Democrats are on 11.7%, with UKIP on 11%, the Greens on 4.1% and the SNP on 3.8%. The strength of the Election Forecast UK model is that it factors 90% uncertainty levels into the equation, in non-stats language, this means that there is substantial statistical uncertainty included in the model. Still, these 90% uncertainty levels provide robustness checks in showing lower and higher values for vote shares.

Table 1- 2015 UK Parliamentary Election Forecast: % Vote Share

(Election Forecast UK)
 Party Lo Votes Hi Swing
Conservatives 31.6% 34.5% 37.3% -2.4%
Labour 29.7% 32.6% 35.3% 2.9%
Liberal Democrats 9.6% 11.7% 13.8% -11.9%
UKIP 9.0% 11.0% 13.4% 7.9%
Greens 2.9% 4.1% 5.6% 3.2%
SNP 3.3% 3.8% 4.3% 2.1%
Other 0.9% 1.7% 2.6% -1.9%
Plaid Cymru 0.5% 0.6% 0.8% 0.0%


This then leads us to the question of how do the % vote forecasts translate into seat shares in Westminster?

The Conservatives have the highest amount of seats with 281 estimated. Again, 90% uncertainty levels are also provided. When turning to Labour, the model estimates the party as having 267 seats- 14 short of the Conservatives. According to the model, the SNP are on course for 51 seats, with the Liberal Democrats on 26. The most striking change, is the translation of votes into seats for UKIP, with the model predicting that the party will only gain one MP in Westminster. Figures 1 and 2 graphically show the variation in the % forecast vote and the translation into seats across time, whilst including 90% uncertainty levels for the lower and higher values.


Table 2- 2015 UK Parliamentary Election Forecast: Seat Share

(Election Forecast UK) (5th May 2015)
Party Lo Seats Hi Swing
Conservatives 252 281 311 -25
Labour 237 267 296 9
SNP 43 51 57 45
Liberal Democrats 20 26 32 -31
DUP 7 8 10 0
Plaid Cymru 2 4 6 1
SDLP 1 3 3 0
UKIP 0 1 3 1
Greens 0 1 1 0
Other 7 8 10 0

Figure 1- 2015 UK Parliamentary Election Forecast: % Vote Share

graph 1

(Election Forecast UK) (5th May 2015)

Figure 2- 2015 UK Parliamentary Election Forecast: Seat Share

figure 2

(Election Forecast UK) (5th May 2015)


General Election Prediction

“Nate Silver, the world’s most respected pollster, says that the Conservatives will win the most seats, but no political party is likely to have enough MP’s to form a majority.”

The renowned pollster Nate Silver recently outlined the difficulty in predicting the 2015 British General Election. Labour still lie behind the Conservatives and the electoral threat posed by UKIP appears to be dissipating. However, in some key marginals UKIP could conceivably stop the Conservative Party from winning and this raises the distinct possibility of a Labour-SNP alliance.

Current election forecasts also outline that the Conservatives will win the most seats and how the ‘shy Tory’ factor must also be considered and has been embodied by “the historical tendency for the Conservatives to do better in the election than in the polls”. Therefore, every vote counts and there is all is to play for in the ground war on the final day of the election.

Note: The author would like to acknowledge for providing access to the data which has been referenced fully.

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