General Election 2019: The polling average might show a Labour surge, but the polls themselves don’t It’s a complicated picture, but the apparent trend is based on some quirks of timing

Ever since Parliament officially called an election, the polling averages have apparently started to move in a very similar way to how it did in 2017, with the Labour average in particular looking like it’s speed-climbing a cliff. Inevitably, that’s prompted much excitement among Labour supporters that 2017 is happening all over again.

Britain Elects has one of the more popular averages, and its most recent iteration shows a clear surge for Labour since 31 October.

However, it’s not really clear that this actually is as much of a surge as it looks. Polling averages are a generally bad way to look at polls. They have methodologies that are just as opaque as regular polls, and often conflict with each other. There’s as much as a 3 point gap between different executions right now, which demonstrates the challenges of agreeing on something as simple as an average when it comes to polling.

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The major takeaway, however, is that the polls themselves aren’t actually moving.

Polling averages work by combining every poll that comes out, usually limited to those conducted by members of the British Polling Council. That’s a long list made up of 13 different pollsters. Some of those have polls in the field every few days during an election, others poll far less frequently. Once the election had been called, a lot of pollsters were asked to do work fairly quickly. That gives a much wider range of polls than would usually be available in a given week.

Each poll has a different character, which is known as a “house effect“. That means it tends to be better or worse for certain parties, and though there’s no way to define it perfectly, we tend to have some grasp of it. One investigation found, for example, that YouGov is largely right for the Tories, but undercounts Labour by about 2.4 per cent. Panelbase does roughly the opposite, overcounting Labour by about 2.3 per cent.

It’s caused by two main things – the people selected to answer, and how those people are modelled. For the first issue, a simplified example might be that one pollster makes calls between 8am to 5pm, and another makes calls between 10am and 7pm. A slightly different group of people are likely to answer the phone at those times, making a small change in the kinds of people included.

Those responses are then modelled, which means different estimates enter the situation. One area this often happens in is “Don’t Knows“, people who say they don’t know who they’re voting for but many of whom actually end up voting. In 2017, the “Don’t Knows” came out fairly heavily in favour of Labour, while in previous elections they either broke out for the Conservatives or evenly between the parties. Each pollster uses the best information available to calculate this, but as their information differs, the end result differs too.

There’s no real reason to think more regular polling makes it more accurate, but what it means in this case is that pollsters who canvass opinion more regularly – defined as those who’ve had two or more polls in the field since the election was called – are coming up with results that aren’t so good for Labour, while those who’ve conducted only one poll have results that are more positive for the party.

In fact, the average gap between the Conservatives and Labour for regular pollsters is 11.1 points, while for less regular pollsters it’s 8.4 points. While that’s only around 3 points, the glut of tighter polls released since the election was called has caused the averages to tighten in a way that isn’t represented in the polls themselves.

We can see this by looking at the polls based on multiple pieces of fieldwork done since the election was called, which show there’s no closing of the gap. YouGov, with five polls, has been up and down, but ultimately the gap has increased by 2 points. ICM has an increased gap of 1 point, ComRes of 3 points, and Deltapoll has remained static at 12 points. Opinium is the only poll with a change outside the margin of error, a closing of 4 points, though it has also been much more volatile over the last few months.

There are still some things we can say with reasonable confidence, however. Both parties have gathered voters since the election was called, but at a similar rate, so Labour’s overall position has improved since the beginning of October. Labour is also in a better position than it was at the same time in the last election, as the Conservatives started from a much lower base.

It’s also worth remembering that the impact of the Brexit Party standing down hasn’t been included in all the polls, and is being managed differently by different pollsters.