How the EU referendum poll tracker works
- Published
How are opinion polls carried out and how can they tell us what the country will decide in the 23 June referendum?
How can polls tell us what large numbers of people may be thinking?
Polling a sample of the population has often been likened to tasting soup: if it is well stirred then you need to have only one spoonful to tell what the whole bowl is like.
In the same way, a well conducted poll of 1,000 people can, in principle, give us an idea of what the country as a whole is thinking.
However, there are several problems that pollsters need to overcome to have a chance of accurately reflecting the whole electorate.
Margin of error
Firstly, no poll can be 100% correct 100% of the time.
Polling companies generally claim that 95% of the time, a poll of 1,000 people will be accurate within a margin of error of +/-3%.
This means that a figure in the poll could be up to three percentage points higher or lower than that shown.
So if "Leave" is on 32% and "Remain" 38%, there is a chance they could both be on 35%.
It is, however, more likely that the figures will be 1% out rather than 3%.
Weighting
Pollsters use weighting to try to ensure their samples represent the general population of the UK.
At its most basic level, this means that if a poll of 1,000 people is made up of 550 men and 450 women, it is unrepresentative because it does not reflect that the UK population is 51% female.
So the answers of female respondents will be given slightly more weight (in this case they will each count as 1.133 people) to give them a representative impact on the final findings.
Conversely, the men will be weighted to each count as 0.891 people.
The same procedure is routinely carried out for age group, social class and region.
Previous voting record
Some pollsters also weight data by previous voting records, by asking respondents how they voted in the May 2015 General Election.
This is done to try and make the sample more closely match the political make-up of the whole population.
This is not without problems though. Some people may not remember who the voted for and a few will even lie about it.
Likelihood to vote
Most companies also weight or filter by likelihood to vote so that the answers of people who are most likely to vote are given the most prominence in the results.
This does have the effect of reducing the number of people on whose answers the final voting intention figures are based - which in turn raises the effective margin of error.
Rounding
Polls are often rounded, so occasionally a result may not add up to exactly 100%.
Polls included
The poll tracker includes polls based on either the actual referendum question, or very similar.
The referendum question is: "Should the UK remain a member of the European Union, or leave?"
The poll tracker currently includes the following companies, which are all members of the British Polling Council.
More detailed methodology information is usually available on their websites.
The poll tracker does not include any polls not based on the wording of the referendum question, or polls carried out on behalf of political parties or groups associated with either side of the campaign.
Timing of publication
The date for each poll is the final date on which fieldwork was conducted.
Poll displayed
Where multiple polls have the same fieldwork completion dates, no value is shown. The data for all polls can be found in the drop-down table.
Headline figures
The poll tracker displays the numbers reported as the headline figure, or identified as the key figures, by each pollster.
Sometimes pollsters produce other figures showing results for those certain to vote, or exclude "don't knows" responses, and these figures may be used or referenced in other media outlets.
"Don't know"
Some polling companies combine their "don't know" and "won't vote" figures, others separate them out.
For some polls shown on the poll tracker, the "don't know" figure also includes "won't vote" responses.