In the run up to polling day last Thursday, a large number of opinion polls were carried out, all aiming to estimate the public’s voting intention. The exact wording of the questions posed by polling companies varied, as did their approaches to weighting the responses, but in essence they all attempted to estimate the share of the vote each major party in Great Britain would receive at the General Election.

The results of these polls were broadly consistent throughout the campaign. But they were so different from the actual result for the two largest parties that the British Polling Council has launched an independent enquiry into the causes of the error.

So how different were the polls from the actual result? The chart below shows seven-point moving averages of the share of the vote for each party as estimated in opinion polls carried out by several major polling companies during the campaign period (from 30th March 2015), with the result of the 2015 General Election marked at the end of each series with a party-coloured X.

Chart showing a moving average of the share of the vote for each major party as estimated by opinion polls during the 2015 General Election campaign

Note: The chart shows the result of all voting intention opinion polls carried out in Great Britain by YouGov, Populus, Survation, Opinium, TNS, ComRes, SportingIndex and ICM from the 30th March 2015 to polling day on the 7th of May. The series are smoothed with 7-point moving averages. The shares of the vote won by each party at the General Election within Great Britain are marked with an X. Sources: May 2015, Electoral Calculus, House of Commons Library: General Election 2015.

Throughout the campaign period, just one opinion poll showed the Conservatives winning a share of the vote greater than or equal to the actual result they achieved (37.6%). This was completed by ICM on the 12th of April and it estimated a Conservative vote share of 39%. However, none of the polls conducted during the campaign period showed a Conservative lead over Labour as large as they achieved (6.5%).

A closer consideration of the final polls

Comparing the final opinion polls, as reported by the British Polling Council, with the final General Election result gives the following differences.

Chart showing the difference between the average of the final opinion polls and the actual share of the vote for each party at the 2015 General Election

Note: This chart shows the difference between the share of the vote suggested by the average of final voting intention opinion polls (carried out by selected polling companies) and the 2015 General Election result, as reported by the British Polling Council. Sources: British Polling Council, House of Commons Library: General Election 2015.

The difference between the share of the vote estimated by opinion polls, and the actual share of the vote received by any party was largest for the Conservatives. Final pre-election opinion polls suggested a Conservative share of the vote 4.2% smaller than that which they went on to achieve. Final pre-election opinion polls most accurately predicted the result for the UK Independence Party: on average they were only 0.1% higher than UKIP’s actual 7th May result.

How does this compare with other general elections?

The next chart shows how final voting intention polls carried out by IpsosMORI over the past 23 years have compared with subsequent polling day results. IpsosMORI is shown here because it is a major polling company with a long historic record of estimating voting intention at general elections. The chart shows the difference between the percentage share of the vote for each party in the IpsosMORI poll closest to each general election and result of that election.

Chart showing the difference between the final IpsosMORI opinion poll and the actual share of the vote for each party at general elections from 1992 to 2015

Note: The number of days between the final IpsosMORI opinion poll and polling day varied from year-to-year. Sources: British Polling Council, House of Commons Library: General Election 2015, House of Commons Library: UK Election Statistics

The difference between the final opinion poll and the result has varied for each party.

Table showing the difference between the Conservative share of the vote in IpsosMORI's final polls and in the actual election result at general elections from 1992 to 2015

Since 1992 the Conservative Party won a higher share of the vote at each general election than the final poll suggested. This was most notable in 1992, when their share of the vote at the election was 4.8% larger than in the final poll.

Table showing the difference between the Labour share of the vote in IpsosMORI's final polls and in the actual election result at general elections from 1992 to 2015

With the exception of 2010, Labour won a smaller share of the vote than the final poll estimated at each general election since 1992. The final opinion polls with largest differences from the election result were those of the 1992 and 2015 General Elections, which estimated Labour’s share of the vote was 3.8% larger than achieved at the election.

Table showing the difference between the Liberal Democrat share of the vote in IpsosMORI's final polls and in the actual election result at general elections from 1992 to 2015

Most recently, the Liberal Democrat share of the vote was 0.1% larger than suggested by the final poll. The largest deviation a Liberal Democrat share of the vote had from a final poll occurred in 2010 when their share of the vote at the election was 3.4% smaller than the final poll estimated.

Table showing the difference between the share of the vote for other parties in IpsosMORI's final polls and in the actual election result at general elections from 1992 to 2015

The ‘Other parties’ category accounts for the share of the votes received by all except the Conservative, Labour, and Liberal Democrat parties. Most recently, the share of the vote achieved by other parties was 2.0% higher than the final IpsosMORI poll suggested.

As the above analysis shows, polling companies have often had a particular difficulty accurately estimating the share of the vote for the Conservatives and Labour. And this bias was particularly pronounced in 2015. The day after polling day the British Polling Council announced an industry-wide investigation into the causes of this error:

The final opinion polls before the election were clearly not as accurate as we would like, and the fact that all the pollsters underestimated the Conservative lead over Labour suggests that the methods that were used should be subject to careful, independent investigation.

The British Polling Council, supported by the Market Research Society, is therefore setting up an independent enquiry to look into the possible causes of this apparent bias, and to make recommendations for future polling.

British Polling Council, General Election: 7 May 2015, 8th May 2015

Nambassa Nakatudde