A report has concluded that the spectacular failure of pollsters to predict the result of the 2015 UK Parliamentary elections was largely due to
systematic over-representation of Labour voters and under-representation of Conservative voters
The report, compiled by a panel of academics and statisticians, was commissioned by the polling industry to determine why they had predicted a “photo-finish” in an election where Conservatives outpolled Labour by 36.9% to 30.4%—a crushing defeat for Labour that lead to the resignation of their leader.
Pollsters apparently used collection methods that were more likely to be used by young (Labour-leaning) voters than older (Conservative-leaning) voters. Frankly, not realising that online surveys are going to under-represent the over 70s is a shocking oversight.
While betting markets also under-estimated the extent of the win, they did better than the polling industry—without the expense.