A long range forecast for the UKIP share of the vote at the 2015 general election

Previous posts at this blog introduce my long-range general election forecasting model, which is updated on Fridays here.

Thus far I haven’t published a forecast of the UKIP share of vote because the party hasn’t been competing in general elections for long enough to build a proper statistical regression model of the relationship between their support in the polls and their votes at elections, as for the three main parties. But the forecast has nonetheless been suggesting a combined ‘Others’ share of the vote around 17% at the next election.

This estimate is naturally heavily dependent of the current fortunes of UKIP in the opinion polls. So there is a case for estimating the UKIP share of the vote in 2015 using their current share of the combined Others share in the polls. This will now be a regular feature of the published forecast.

As you can see from the current forecast below, UKIP are at 12% in the current UKpollingreport.co.uk polling average. This is 60% of the combined Others total of 20 percentage points. Since the current forecast for all Others combined is 17.0%, the UKIP forecast share is 0.6*17 = 10.2%.

I still haven’t decided how best to estimate prediction intervals for the Others and UKIP, but they are likely to be of a similar size to those for the main parties, i.e. plus or minus 8 percentage points.

Despite forecasting a substantial increase in the share of the vote for UKIP, the model is not forecasting any seats for UKIP in 2015. This is despite attributing all of the increase in the Others share to UKIP wherever it was the largest Other party in 2010. This reflects the very flat distribution of the UKIP vote across constituencies in 2010.

Whether the UKIP vote will be similarly evenly spread in 2015, or whether they will be able to mount successful campaigns in selected seats remains to be seen. It is certainly something that can’t be predicted this far from the election with the statistical technology this forecasting model is based on.

Date of forecast: 04 April 2014
Days till the election: 398

Inputted current average poll shares
Con: 33%
Lab: 37%
LD: 10%
Others (inc. UKIP): 20%
– UKIP: 12% (so 60% of Others)

Forecast Election Day Shares (with 95% Prediction Intervals)
Con: 36.9% (±8.4, i.e. 29% – 45%)
Lab: 31.8% (±6.3, i.e. 26% – 38%)
LD: 14.3% (±9.9, i.e. 4% – 24%)
Implied point estimate shares for:
– Others (inc. UKIP): 17.0%
– UKIP: 10.2%

Forecast Election Day Seats (with approximate 95% Prediction Intervals)
Con: 308 (218 – 413)
Lab: 283 (185 – 368)
LD: 31 (25 – 39)

Central forecast: Con largest party, but short of a majority by 18

Approximate probabilities of key outcomes
Con majority: 35%
Lab majority: 19%
Hung Parliament: 46%
… with Con largest: 25%
… with Lab largest: 21%


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