• Pro-Brexit party underperformed projections in London, Wales
  • Findings suggest Brexit support may also be overestimated

Online opinion polls in the run-up to May 5 elections across Britain overstated support for the U.K. Independence Party, suggesting they may also be overestimating backing for leaving the European Union, according to a polling analyst.

For months, pollsters have been aware of the difference in outcome between online and telephone polling on the question of EU membership. With the referendum on a so-called Brexit coming on June 23, online polls put the two sides neck-and-neck, while telephone polls have the “Remain” side in the lead.

Last week’s elections offered an opportunity to check their findings. According to Matt Singh of Number Cruncher Politics, 10 of the 12 online polls conducted in the two weeks before the vote overstated UKIP support. In polling for the London and Welsh assemblies, the online polls were on average 3-4 percentage points too high.

“Support for leaving the EU and votes for a pro-Brexit party are clearly not the same thing,” Singh wrote on his blog. Still, “online panels seem to be too Leave-y.”

Following last year’s collective failure by pollsters to correctly predict the general election result, the reliability of opinion research companies has been under scrutiny. An investigation concluded there was no easy solution to the problem.

Singh, who argued just before last year’s vote that the polls were understating Tory support, has suggested that part of the problem for Brexit polling may be that online polls have too many social conservatives in their samples.

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