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Last Updated: Monday, 25 April 2005, 14:21 GMT 15:21 UK
Guide to pollsters' methodology
Asking people who they intend to vote for is only the first step in working out the parties' standing in the country.

This is a basic guide to how the polling companies arrive at their final percentages.

METHODOLOGY IN DETAIL

Polling a sample of the population has often been likened to tasting soup: if it is well stirred then you only need to have one spoonful to tell what the whole bowl is like.

In the same way, a well conducted poll of 1,000 people can, most of the time, 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

The first caveat is that 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%.

POLL TRACKER

This means that figure in the poll could be up to three percentage points higher or lower than that shown.

So if the Tories are on 32% and Labour is on 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%.

The question

How the question is framed can have an effect on the results. Companies have to decide whether to simply ask people who they would vote for or to remind them of the choices (as they would have on a ballot paper).

Research indicates that the Lib Dems have a higher rating in polls where respondents are given a list of parties to choose from.

Weighting

Another issue is how to ensure the sample is representative of the general population. To achieve this, polling companies "weight" their data to match the demographic profile 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 the profile of the UK population (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.

Past vote

For voting intention polls, further adjusting is required. Some pollsters weight by past vote - they ask respondents who they voted for last time and weight the sample so that it more closely matches the political make-up of the general population.

One problem with doing this is that a certain number of people will incorrectly recall who they voted for last time - and a few will even lie about it.

So the polling companies that weight by past vote - ICM, NOP, Populus and YouGov - use a variety of further methods to improve the accuracy of their weighting.

These include using information from a range of previous voting intention surveys.

Likelihood to vote

Most companies then 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.

'Shy Tories'

Finally, several pollsters reallocate a percentage of "don't knows" to the party they voted for last time.

This is to get around the problem of "Shy Tories" or the "spiral of silence" - people who do not like to admit they support a certain party but who vote for them nonetheless.

In the past this has particularly applied to the Conservatives but there is some indication that a trend of "Bashful Blairites" is emerging.

Below are more details of the different polling companies' methodology as they apply to their current voting intention polls.

Communicate Research

Question: Which party do you intend to vote for in the general election? (Conservative, Labour, Liberal Democrat, Plaid Cymru/SNP, Green, UKIP, other party)

[Communicate have only recently begun prompting by party name. Only polls after 3 April are fully prompted.]

Interview method: Telephone

Sample size: Approximately 1,000 adults age 18+

Sample method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is demographically weighted to reflect the profile of all adults in the UK aged 18+, including those in households that do not own telephones.

CommunicateResearch weights the data by likelihood to vote. This is done by asking respondents how likely they are to vote (on a scale of 1-10 - with 10 being absolutely certain to vote).

The answers of respondents who answer four or below are excluded. The rest are weighted proportionately so that someone who answers 10 gets the most weight and someone who answers five gets the least.

Respondents who have answered "don't know" to the voting intention question or refuse are asked a further "squeeze" question: if it were a legal requirement for you to vote, which party do you think you probably would vote for?

This extra data is then weighted and added to the original voting intention results.

Communicate does not weight by past vote.

ICM

Question: If there were a general election tomorrow which party do you think you would vote for? (Conservative, Labour, Liberal Democrat, Plaid Cymru/SNP, other party)

Interview method: Telephone

Sample size: Approximately 1,000 adults age 18+

Sample method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is demographically weighted to reflect the profile of all adults in the UK aged 18+, including those in households that do not own telephones.

ICM then weights the data by past vote.

The answers are also weighted by likelihood to vote. This is done using a combination of how likely respondents say they would be to vote in a new election (on a scale of 1-10) and how consistent they have been at voting in the past.

In a further step, ICM takes 50% of those who intend to vote and have a good voting record but are undecided who they will vote for this time and allocates them to the party they say they voted for in 2001.

MORI

Question: How would you vote if there were a general election tomorrow? (Conservative, Labour, Liberal Democrat, Other)

Interview method: Face-to-face or telephone

Sample size: Approximately 2,000 adults aged 18+ face-to-face; approximately 1,000 adults aged 18+ by telephone

Sample method: MORI conducts face-to-face polling at about 200 selected points in constituencies around Great Britain which are specifically selected to be representative of the country as a whole.

The sampling points within each selected constituency are picked to be representative of that constituency. Respondents are selected on a quota basis to reflect the demographic make-up of the general population.

In telephone polls, MORI uses random digit dialling so as to include households with ex-directory numbers.

Weighting: Data is further weighted to reflect the general population, using the latest census data and recent updates from the Office for National Statistics and other surveys.

MORI asks respondents to rate their likelihood to vote on a scale of 1-10 where 10 means absolutely certain to vote. The data is then filtered so that only the responses of those who answer 10 out of 10, absolutely certain to vote make up the final figures.

Those who are undecided or refuse to answer are excluded.

MORI does not weight by past vote.

NOP

Question: If you do vote in the next general election, which party will you vote for? (Conservative, Labour, Liberal Democrat or some other party)

Interview method: Telephone

Sample size: Approximately 1,000 adults aged 18+

Sample method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is weighted to reflect all adults in the UK aged 18+, including from households that do not own telephones.

NOP asks respondents to rate their likelihood to vote on a scale of 0-10 where 10 means absolutely certain to vote.

Their responses are then weighted proportionately - e.g. someone who answered 6 on the scale would be given a weight of 0.6 whereas someone who answered 10 would be given a weight of 1.

NOP then weights the data by past vote.

Those who say they would not vote and don't knows are excluded from the final figures.

In polls published since 5 April 2005, NOP has made a further adjustment to offset the "spiral of silence".

Populus

Question: If the general election was tomorrow which party would you vote for? (Conservative, Labour, Liberal Democrat [rotate order], another party, or would you not vote at all?)

Sample: Approximately 1,500 adults 18+

From 20 April 2005 Populus have been running a daily tracker poll in which a quarter of the sample is replaced each day. A more detailed explanation of the methodology involved can be found on the Times Online.

(See Poll tracker FAQs for an explanation of how we are representing the tracker.)

Sampling method: Within each region of the UK a random sample of telephone numbers is drawn from BT's domestic database. The last digit is randomised so that unlisted numbers are also hit.

Weighting: Data is weighted to the profile of all adults aged 18+ (including non telephone owning households).

The data is further weighted on the basis of past vote.

Figures are adjusted for turnout on the basis of respondents' declared likelihood of voting.

In a further step Populus takes 60% of those who say they are going to vote at the next election, but who also refuse to answer the vote intention question, or say they don't know how they'll vote, and allocates them to the party they voted for in 2001.

YouGov

Question: If there were a general election tomorrow, which party would you vote for? (Conservative, Labour, Liberal Democrat, SNP/Plaid Cymru, some other party, would not vote, don't know)

Interview method: Internet panel

Sample size: Approximately 2,000 adults aged 18+

Sample method: YouGov has recruited an online panel of about 70,000 subscribers. From this panel YouGov selects a sub-sample to match the UK electorate by age, gender, social class and type of newspaper read. Only this sub-sample has access to the questionnaire. Respondents are paid for taking part.

Weighting: Data is weighted to the profile of all adults aged 18+ including people without internet access.

As well as weighting by gender, age, social class and region, YouGov weights by readership of individual newspapers.

Data is also weighted by past vote. To do this, YouGov uses estimates derived from comparing voting intention of 5,000 respondents at the time of the 2001 election with the responses from the same panel members to later questions asking them to recall their 2001 vote.



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