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In early October, when this site was only a few weeks old, we looked at how Labour were heading into the election year with a 3-4 point lead in the polls.

Almost every seasoned political pundit doubted that lead would last. So did almost every Tory supporter. And, most importantly, so did the election forecasters who had started to predict the election.

In the four months since then, Labour’s lead has undeniably shortened. In September the party were consistently polling 35-36 per cent. Now they typically poll around 33 per cent. Yet it’s hard to say that they have actually lost their lead.

This afternoon Lord Ashcroft put the Tories ahead by 3 points, which is set to spark a mini-news cycle. But we’ve been here before. Today’s poll is the ninth to put the Tories ahead in 2015 – out of 54. Most polls (33) have put Labour in front.

For the past two months May2015 has often suggested the election race is tied – with both major parties regularly exchanging poll leads.

After a trio of polls put the Tories fractionally ahead a fortnight ago, Labour once again regained a small lead. That lead has just about held up after today’s Ashcroft poll.

The Tories’ long-awaited rise in the polls has been inconsistent at best. The party has been stuck in a 30-33 per cent band for three years, and it’s unclear why that would suddenly change.

The economy started recovering nearly two years ago, and David Cameron has long been far more popular than Ed Miliband.

If the economy and leadership are going to hand the Tories the election, why hasn’t it happened yet?

This is the question Helen Lewis, the New Statesman’s deputy editor, asked last week. It’s encouraged us to revisit the statements made by Stephen Fisher, the Oxford forecaster, whose work seems to have inspired many pundits.

In early 2014 Fisher looked at whether the polls have tended to move in “systematic ways” before previous elections. He decided that there were three historical trends:

First, the two main parties are more likely to increase support in the run up to the election when in government than in opposition.

Second, the Conservatives typically outperform and Labour under perform the opinion polls.

Third, if a party is doing unusually well in the polls some way before the election their support is likely to drop and vice versa.

“These three factors all help explain why the Conservatives are expected to do a lot better at the election and Labour worse than the polls currently suggest,” he concluded.

The idea that Tories gain, and Labour fade, before polling day has been less true at each election since 1983.

Fisher’s understanding of historical election data exceeds this website’s, but the data was never as conclusive as Fisher’s conclusion arguably made it sound.

His central theory – that the Tories will gain, and Labour fade, relative to the polls twenty months before an election – was based on the past seventeen elections. But this truism has been less true over each of the past seven elections.

Since 1983 the traditional swing back to the Tories was lower at every election cycle. In 2010 the Tories actually faded in the polls relative to the final result. And in 2005 and 2010 Labour didn’t fade, as the long-term trend suggests.

The wider point in Fisher’s paper is similar to the one recently made by Chris Hanretty on May2015. Fisher explains how polls are more effective at predicting final election results than other metrics like PM approval, citizen forecasts or local elections.

But even the polls have limits. As Hanretty showed, eve-of-election polls only predict 80 per cent of the final election result, and they only predict around 60 per cent of the result three months out from the election.

Fisher’s model has a similar principle at the heart of it. Both his model (Fisher runs Elections Etc) and Hanretty’s (Hanretty is part of the team behind Election Forecast) take the current polls and discount them against the 2010 election. In other words, they start with the 2010 result and then apply a fraction of the change implied by current polls.

The forecasts made by Elections Etc and Election Forecast are pegged to the 2010 election.

That pegs their predictions to the 2010 election. Our model, on the other hand, is more of a “nowcast”. It uses a rolling two-week average of the polls to project show how the parties would fare in an election held today. Despite this, our forecasts are very similar: we all project Labour and the Tories will fall far short of a majority, and win about as many seats as each other.

But Fisher and Hanretty are both predicting the Tories will win the popular vote, because of the way their models hark back to 2010.

They are justifiably relying on historical precedent, but that precedent is limited. We are relying on a handful of data points, and are using an average of them when individual (and recent) results have contradicted these supposed “norms”.

It’s also unclear whether we can really apply ideas like “swing-back” to the rise of Ukip or the SNP. The big differences in election forecasts is over predictions for the SNP: forecasts range from the mid-30s to the mid-50s (there are 59 seats in Scotland).

It’s unclear whether we can really apply ideas like “swing-back” to the rise of Ukip or the SNP.

That is primarily because Fisher and Hanretty are discounting the current polls against the 2010 election, when the SNP only won 20 per cent of the vote. But in post-referendum Scotland, such trends may not apply.

The Tories can’t reassure themselves. There is no guarantee the polls will tighten. The party needs to “squeeze” Ukip, who have taken around a fifth of their 2010 voters.

Many pundits think this likely, but few have much experience of a Ukip-style revolt. The only precedent is the rise of the Liberals/SDP/Lib Dems, and those movements rarely faded before an election. In 1974, 1983 and 2010 Britain’s liberals rapidly rose ahead of each election and held up well until polling day.

In 1983 the SDP didn’t top the poll, as it looked like they might in the winter of 1981, but they took a quarter of the vote. There was far more votes to be “squeezed” then, and in 2010, than there are for the Tories to squeeze now.