Off Target

Did weightings induce a polling error for one company in 2019?

ICM Unlimited conducted voting intention polls for the 2019 election. Their final election poll underestimated the Conservative lead by six points. In Great Britain, the actual lead was around 12 points.

This article considers if their weighting targets induced an error.

Final polls

ICM Unlimited collect internet panel responses, for Great British adults. Their last vote intention poll was for 6–9th December 2019. In final polling, the mean average estimated Conservative lead was nine points.

Savanta ComRes estimated the smallest lead of all companies. (Image: Google Sheets)

There are house effects. Relative to the average company, ICM Unlimited’s Conservative estimates were lower. ICM Unlimited estimates for Labour were higher. Among final polls, their central Labour vote intention estimate was joint-highest, at 36%.

ICM Unlimited were consistent — showing a narrower lead than the average company. There were method changes too, during the campaign.

The importance of age

ICM Unlimited are transparent about their methods. As the British Polling Council requires, the company states:

The data has been weighted to the profile of all adults aged 18+ in Great Britain and is weighted by…