How did pollsters do in predicting the British election?

An average of 17 voting-intention polls conducted immediately before election day suggested that Labour would enjoy an 18-percentage-point lead over the Conservatives. But once the ballots had been counted, Labour’s share of the vote was just 10.3 points ahead of the Tories’. Underestimating the Conservative Party’s support and over-estimating Labour’s is an age-old problem for the polling industry. But this is the biggest miss since the election in 1992. Jane Green, a professor of politics at Oxford University and president of the British Polling Council, says it is “too soon to know why” the pollsters were off target. Even so, theories abound.