Category Archives: Methodology

After a year of forecasting, what’s changed?

Stephen Fisher, 24th October 2014

I first published a forecast of the 2015 general election result in October 2013. After taking on board comments and testing more candidate models, in February I revised the method from the one in this working paper to the one in this working paper. Both use opinion polls and election results going back to the 1950s to tell us what is likely to happen in this electoral cycle, and importantly, how sure we can be that it will happen. The historical pattern suggested governments tend to recover from mid-term blues while oppositions suffer a set back. Also the polls have tended to overestimate Labour and underestimate the Conservatives. Both factors suggested a Conservative lead at the next election. But also the variation in previous cycles was plenty enough to suggest very different outcomes were also possible if less likely.

Using a polling average for 8th October 2013 of Conservatives 32%, Labour 39% and Liberal Democrats 10% the revised method suggested a 42% chance of a Conservative overall majority. Over the last year the probability of a Conservative majority dropped steadily to 24% now. Why? Continue reading After a year of forecasting, what’s changed?

How accurate will the Scottish independence referendum polls be?

Stephen Fisher, 11th September

The vote intention polls for the Scottish independence referendum seem to have been taken largely at face value by commentators, politicians and even the financial markets. In particular, the roughly equal split of the vote between Yes and No in several recent polls is being interpreted as if it implies that the result of the referendum is likely to be close. But how accurate are opinion polls as predictors of referendum outcomes and how accurate are polls for elections in Scotland more generally? Continue reading How accurate will the Scottish independence referendum polls be?

Extreme events and probabilistic election forecasting: salutary lessons from football

In his article on last week’s forecast, John Rentoul wrote:

“Probability is hard enough to understand anyway, of course. Look at Nate Silver, the guru of American election predictions. He said Brazil had a 65 per cent chance of winning against Germany in the World Cup semi-final. Well, you could say that their 7-1 defeat fell in the other 35 per cent but – after the event – we can be pretty confident that the 65 per cent figure meant little useful.”

There are various issues here in the context of the overall article. Did the 65% figure mean little useful? How should we judge probabilistic forecasts after the event they were trying to predict? Even if John Rentoul’s interpretation here is right, should the poor performance of a football match prediction undermine the credibility of other forecasting exercises of very different kinds of events for the forecaster, or even for all forecasters? Continue reading Extreme events and probabilistic election forecasting: salutary lessons from football

Fluctuations in polls and the timeliness of the forecast

Belated thanks to John Rentoul for his excellent article on last week’s forecast. This is not to say that I necessarily agree or disagree with his views on the desirability of particular election outcomes, just that I think it is a good discussion of lots of issues in politics and forecasting. Continue reading Fluctuations in polls and the timeliness of the forecast

Why is the 2015 general election forecast trending?

A few people have asked me this question about my forecast. The trends are definitely there as you can see from the graphs below. But since they are well within the very broad prediction intervals, there is a danger of reading too much into them. Certainly we are far from having enough information to say the model isn’t working well for this electoral cycle. Continue reading Why is the 2015 general election forecast trending?

What do the 2014 European and local election results mean for the opinion polls and next year’s general election?

Labour emerged narrowly ahead of the Conservatives in both the local and European Parliament elections. I discussed the implications of the local election results in a previous post on Friday. The results of the Euros only came through last night. This post considers the lessons learnt from both elections for the general election, including the likely accuracy of the opinion polls. Continue reading What do the 2014 European and local election results mean for the opinion polls and next year’s general election?

Reasons for changing the forecasting model: a response to The Economist

The Economist magazine have published a great article on the difficulties of predicting the next election. It makes lots of good points well and kindly covers my 2015 general election forecasting model. But I was bemused by the line that reads, “Within months he had published a revised model: polls had not adhered to the original one.” Continue reading Reasons for changing the forecasting model: a response to The Economist

How the 2015 general election forecast probabilities have changed thus far and why

The graph below shows how my 2015 general election forecast probabilities have changed since October last year.* The blue line shows the probability that the Conservatives will have the largest number of seats. The corresponding red line for Labour is just the mirror image. Also included in the graph is the probability of a hung parliament. Continue reading How the 2015 general election forecast probabilities have changed thus far and why