{"id":2264,"date":"2019-12-29T23:22:10","date_gmt":"2019-12-29T23:22:10","guid":{"rendered":"https:\/\/marriott-stats.com\/nigels-blog\/?p=2264"},"modified":"2021-01-19T12:53:36","modified_gmt":"2021-01-19T12:53:36","slug":"uk-general-elections-4-how-accurate-are-the-polls-updated-with-ge19","status":"publish","type":"post","link":"https:\/\/marriott-stats.com\/nigels-blog\/uk-general-elections-4-how-accurate-are-the-polls-updated-with-ge19\/","title":{"rendered":"UK General Elections #5 – How accurate are voting intention polls? – updated with GE19"},"content":{"rendered":"

After 3 general elections with severe polling errors, the UK opinion pollsters redeemed themselves in the 2019 UK General Election<\/a> with their most accurate performance since 1955.\u00a0 I base this statement on data provided by Mark Pack who has systematically recorded every opinion poll published since 1945<\/a>.\u00a0 The challenge now for the industry is to maintain this level of performance for the next election which may be easier said than done given that 5 out of the last 8 elections have experienced a major polling error.<\/p>\n

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My analysis looks at only the polls that took place in the week before every general election between 1950 & 2019.\u00a0 Note this is based on the fieldwork dates, not the publication date which can be a few days later.\u00a0 For the elections in the 1950’s, Gallup were the only pollster so instead of using the week before, I used the month before.<\/p>\n

For each election, I calculated the average vote share recorded across all pollsters for the Conservatives, Labour and Liberal Democrats (Liberals 1950 to 1979, Alliance 1983 & 1987).\u00a0 The polling error for each party can then be calculated as the actual election result for Great Britain minus the average vote share from the polls.\u00a0 I use the figures for Great Britain rather than the United Kingdom since nearly all polls do not survey in Northern Ireland.<\/p>\n

The results can be seen in the chart below.\u00a0 The black line is the number of polling companies that polled in the run up to the election.<\/p>\n

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After 3 general elections in a row with major polling errors, the polling industry breathed a sigh of relief in 2019 with all 3 parties predicted to within 2 percentage points.\u00a0 The last time this happened was in 1979.\u00a0 In fact, the pollsters performance is better than 1979 if you measure the RMSE (Root Mean Squared Error) based on the 3 parties.\u00a0 This is calculated by squaring the errors, finding the average of the squared errors and then taking the square root of the average.\u00a0 In 2019, the RMSE was 1.1% which is the lowest seen since 1955 and the second lowest on record.<\/p>\n

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Has the polling industry overcome the issues it has had in the 3 elections prior to 2019?\u00a0 History tells us that RMSE this low are rare and higher errors can occur.\u00a0 During the election, I kept pointing out that 5 of the last 7 (now 8) elections saw at least one party experiencing a polling error of at least 4 percentage points, namely –<\/p>\n