Why is the 2015 general election forecast trending?

A few people have asked me this question about my forecast at http://users.ox.ac.uk/~nuff0084/ge15forecast/. 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.

In fact, it is more remarkable that public opinion and the forecasts haven't drifted or fluctuated more over the last nine months than it is that the forecasts show a bit of a trend from the initial forecast.  The current forecast is very close to the original one given the width of the prediction interval.

That said, it is worth saying something about the reasons for the trends. Some of them were expected and some are due to deviations from the expected trajectories of the parties in the polls.  I'll start with the latter and then return to the former.

First, then, trends in the forecasts partly reflect trends in the polls relative to their expected trajectories which are based on the average historical patterns in electoral cycles for different parties according to whether they are in government or opposition and their results at the last general election.

The Conservatives and Liberal Democrats were expected to recover some of their mid-term losses over the last nine months. In fact the Conservatives have roughly been level pegging around 32% in the polls since October, so their forecast election share has declined slightly. They haven't made the recovery that history suggests they ought to have done since the autumn.

The Liberal Democrats were steady in the polls at 10% but have dropped a bit to 8% since the local and European elections. So their forecast general election share has declined from 16% to 12%.

Labour, meanwhile, has seen its poll share decline roughly by the average from previous electoral cycles, and so their forecast share of the vote has remained pretty steady around 32%. Arguably by matching expectations Labour have done better than the Tories who failed to meet them over the last nine months.

As a result of a declining Conservative forecast share and steady Labour forecast share the chances of a Conservative lead in votes or seats has drifted down, and conversely the probability of a Labour lead has drifted up in the graph below. It still remains more likely that the Tories will emerge ahead. But 57% probability is not much greater than a half.

The seats prediction without the prediction interval (uncertainty range) has always pointed to a hung parliament.  With the prediction interval there is a slightly better than 50% chance that one or other of the two main parties will emerge with a parliamentary majority. Since October the chances of a hung parliament have increased from 40% to nearly 50%.

This is firstly because the forecast number of Tory seats has dropped (from 315 to 305) but also because the uncertainty around this forecast has declined. There is now a greater chance that the result will be close to the main estimate than there was in October. This is partly because there is less time for change, and so polls 300 days from the election are more informative about the eventual result than polls 600 days before.

This is the sense in which trends in the forecast were expected. It was expected that the prediction intervals would narrow slightly with time and so (without big changes in the seats forecast) the chances of a hung parliament were also expected to increase as the chances of big changes in before the election decline.

While declining, the chances of big changes between now and the election remain substantial. The prediction intervals on the shares of the vote differ by party, but they are still now around plus or minus 7. Plenty of room for both a big Labour or big Conservative lead to emerge.