How we calculated our extrapolated Brexit vote shares

By I.K. | NEW YORK

ELECTION nights usually mean instant gratification: exit-poll results are often available immediately after the last vote is cast. But tonight’s referendum on Britain’s membership of the EU is an exception. Because no similar ballot has been held since 1975, mainstream media organisations decided there were not enough historical data to justify commissioning an exit poll. As a result, the final result will not be known until the losing side is mathematically eliminated. If the vote is close, that will be long after the sun rises on June 24th.

Hours before a winner is declared, however, it will be possible to reach a reasonable expectation of the likely outcome. That is because the data from early-reporting areas contain two separate types of information. In addition to hard vote totals that count towards the result, they also offer precious clues about what to expect from regions that have not yet announced their figures. The Economist has built a simple statistical model that extrapolates the numbers from ballots that have already been reported to produce an estimate of the margin among those that haven’t.