Methodology

Populus is a member of the British Polling Council and abides by its rules. Our commitment is to standards of disclosure designed to ensure that consumers of survey results that enter the public domain have an adequate basis for judging the reliability and validity of the results. In accordance with these rules the following information is provided.

Interview: Method Telephone

Population effectively sampled: All adults aged 18+ 

Sample size: 1,500+

Sampling Method: Within each government office region a random sample of telephone numbers was drawn from the entire BT database of domestic telephone numbers. Each number so selected had its last digit randomised so as to provide a sample including both listed and unlisted numbers.

Data weighting: Data were weighted to the profile of all adults aged 18+ (including non telephone owning households). Data were weighted by sex, age, social class, household tenure, work status, number of cars in the household and whether or not respondent has taken a foreign holiday in the last 3 years. Targets for the weighted data were derived from the National Readership survey, a random probability survey comprising 34,000 random face-to-face interviews conducted annually.

For polls on political issues, the data are further weighted to be as politically representative as possible on the basis of recalled past vote at the 2005 general election – as described below.

Voting intentions: Populus derives voting intention from 4 questions. First of all respondents are asked how likely it is that they will actually vote at the next general election. Those who say they will vote are asked to say which party they would support if there were a general tomorrow. Next respondents are asked whether they voted at the last general election and, if so, for which party.

The voting intention figures are calculated after Populus has excluded those who say they will not vote, who refuse to answer the question or who don’t know which party they would vote for. The figures are then adjusted for turnout on the basis of respondents’ declared likelihood of voting on a ten point scale – so that, for example, if someone says they are ’10 out of 10’ certain to vote their voting intention is counted at full value (1.0), but if they put themselves at 8 out of 10 on the scale their voting intention preference is counted at a value of 0.8.

Populus then weights the whole sample on the basis of its ‘past vote’ - adding the most recent poll data to its previous 20 most recent voting intention polls (so as to avoid the random volatility that can appear in comparing any two individual samples) and calculates the past vote weighting from the average recalled past vote in this data, giving a weight of 50% to the actual result of the last election and 30% to the average recalled past vote from the most recent polls.

An additional step is then taken to address the tendency for ‘spirals of silence’ among supporters of unpopular parties causing an inadvertent bias in voting polls. This is done by taking those respondents who will disclose which party they voted for at the last election, but refuse to answer the question of how they would vote in an election now, or say they don’t know, and reallocating them to the party they voted for at the last election at a value of 0.5 if they voted Labour or Conservative last time, or 0.3 if they voted Liberal Democrat. These values are derived from a 5,000 sample callback poll immediately after the last election, when Populus re-interviewed people polled in the run up to the election, comparing what they actually did with what they said they would do when first interviewed. This final stage of the weighting is done manually and so the ultimate voting intention figures cannot be broken down below the topline. For this reason the detailed breakdown of voting intention shown in the tables is based on the data before the spiral of silence adjustment.

Unweighted and weighted bases are shown at the top of each computer table and such analysis covers all conclusions made in any report.

Further information about the BPC can be found at: www.britishpollingcouncil.org 

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