Regional variation, or RegVar (after the brass band) is our approach to forecasting the 2017 UK General Election. Unlike forecasts based on uniform, or adjusted uniform swings, RegVar attempts to aggregate together data from previous elections to predict how a particular seat will behave relative to the UK as a whole, and to nearby seats.

The aim is to avoid the kinds of systematic polling errors caused by small mistakes in the headline national polls. As Ed Miliband will tell you, more votes doesn’t always mean more seats under first past the post. Similarly, on a grander scale, we saw Hillary Rodham Clinton rack up a two million popular vote lead while losing the electoral college – she would have needed her voters to be less than 2% more efficiently distributed to win the day, and the presidency.

RegVar works first by taking the national vote share and comparing it with the regional vote share, simulating the unequal distribution of votes. In the South East, you would expect the Tories to pick up twelve votes for every ten they received nationally, while they might get only seven in the North West. Consequently, for every 100 Tory votes nationally, we expect perhaps 120 Tory votes in the Guildford.

From this, we can see how each seat varies from the regional pattern – Brighton Pavillion, Caroline Lucas’s seat – has one of the biggest gap between the regional vote pattern, roughly 51% Tory, and the seat level vote outcome, 23% Tory. These disparities are the basis for RegVar, which handicaps each party in each seat against historical, national, and regionally adjusted performance.

In practice, this allows us to suggest what kind of task each party is up against in each constituency. In Maidenhead, the Labour party was 7 points behind its overall South East of England performance in 2015, and is hardly likely to make up ground now. Meanwhile in Gower, a seat the Tories won by 27 votes, generous polling is making the country’s slimmest majority look unusually comfortable.

We have also incorporated the results of the EU referendum into our model, which we are calling the Brexit Bonus. The Brexit Bonus further adjusts the overall handicap of each seat depending on the Leave margin, and the competing parties’s stances, which hopefully will replicate, at least in spreadsheet form, Tim Farron’s hopes and dreams.

We are working to improve RegVar over the coming weeks, incorporating local and mayoral election results, turnout variables, and long-term polling patterns from the archives, going right back to 1970, as well as the exit polls on the night itself. RegVar does not pretend to tell you who will be the next Prime Minister – we all know who she is – but we do hope to give you a sense of just how bad, or good, things could get.