And the crowd says… Shutterstock

UK election 2019: what the ‘wisdom of crowds’ forecasts

Forecasting outcomes in 21st-century politics can sometimes feel like a fool’s undertaking. The media landscape is crowded with conflicting interpretations and analyses of campaigns and voting intentions – and the failure of many polls to forecast the outcome of the 2015 UK general election in particular has led to each new poll’s accuracy being hotly debated in a way that was unthinkable a decade ago.

So how else can we cut through the noise to build a more objective assessment of what is likely to happen on December 12? We argue that it’s worth considering crowd forecasting as an alternative to more traditional methods. Why? Because the crowd has proved to be right about Brexit so far.

Earlier this year, we wrote about the ability of an amateur, global crowd to accurately forecast the extension of Article 50 in March and the vote share gained by Change UK and the Brexit Party in the May 2019 European parliament elections. In retrospect, these outcomes may seem obvious and inevitable, but at the time, uncertainty was rife.


Read more: Brexit: wisdom of crowds proves effective predictor of Britain's chaotic EU departure


At Nesta’s Centre for Collective Intelligence Design, we’ve been carrying out a year-long experiment to put crowd wisdom to the test in the face of the high uncertainty caused by Brexit. We’ve been working on this with BBC Future and the online forecasting platform Good Judgment Open, which first showed that a crowd of non-specialists could make surprisingly accurate estimates of probability about geopolitical events.

When it comes to forecasting unexpected election outcomes, there is some precedent. Although Good Judgment forecasters didn’t anticipate a Donald Trump victory outright, they were the least wrong compared to other prediction methods. Indeed, crowd-based prediction markets have even been shown to outperform intelligence analysts.

Since the summer, our crowd has continued to perform strongly on Brexit questions. For example, it correctly decided an October Article 50 extension was the most likely outcome from a range of options (which included leaving without a deal and leaving with a new deal), and accurately forecast what the value of the pound versus the Euro would be on November 1, 2019.

We’re now monitoring their evolving forecast for what will happen in the UK general election.

What is crowd forecasting?

The process is based on the “wisdom of crowds” principle. When large groups of distributed individuals are all asked to make estimates on numerical problems, they rely on unique personal skills and sources of information. They also tend to make errors in different ways – for example, some may have a tendency to overestimate while others do the opposite. This means that when you aggregate and average across all of the estimates, their errors cancel out, making them more accurate than individuals on most occasions.

But that’s not all. Our crowd forecasting also promotes the open sharing of information between participants and allows the adjustment of estimates over time. This makes it different from other popular approaches to predicting election outcomes such as opinion polling and prediction markets.

‘They said that?’ Shutterstock

Participants are incentivised to update their forecasts as new information becomes available, in order to improve their personal “Brier score” (which measures the accuracy of their performance). It’s been shown that this approach helps to promote a more objective assessment of evidence from different sides of the political spectrum, in contrast to the reported polarising effects of social media bubbles.

Forecasting the election

Most recently, we’ve been asking our forecasters to estimate the likelihood of a single party majority in the House of Commons. In the first half of November, we saw the option of a “Conservative majority” battle it out with “No overall majority”. But this changed from November 14 when the crowd first assigned the decisive highest likelihood to the Conservatives having a majority.

Since then, the consensus estimate has consolidated. It currently assigns a 75% likelihood to a Conservative majority, while the likelihood of a Labour majority is now 0%. The likelihood of a hung parliament is at 25%.

We’ve also asked our crowd to predict how many of ten key marginal seats will change hands. Currently they’re forecasting that five or six of the ten will have new parties representing them.

What the crowd has forecast over time. Nesta., Author provided

Elsewhere, the last week has seen reports of Labour closing the polling gap and record numbers of young voters registering in key marginal seats. Prediction markets have seen traders adjusting to these media reports with a slight uptick in the likelihood of “no overall majority” over the last week.

Noted political commentator Sir John Curtice has also warned that recent changes to the balance between Liberal Democrat and Labour support may affect the Conservatives’ chances of a majority. This contrasts with the prediction issued by YouGov’s much lauded MRP model that has estimated a 60+ seat majority for Conservatives.

So will the crowd manage to cut through the Brexit noise once again? We’ll see in seven days. In the meantime, you can register your own forecast by signing up on the Good Judgment platform.