SOURCES
The FT’s main UK poll tracker uses data from every new national voting intention poll by a member of the British Polling Council. Voting intentions in demographic subsamples use data from Deltapoll, Ipsos, Omnisis, Opinium, PeoplePolling, Redfield and Wilton, Savanta, Survation, Techne and YouGov.
METHODOLOGY
We use general election polls covering the United Kingdom or Great Britain to calculate both an average voting intention and a range of likely values for each party.
To compute the average at a particular moment in time, we take the most recent poll from each pollster and assign it a weight based on its sample size and how long ago it was released. Polls released today are weighted the most, while polls released more than 30 days ago are not weighted at all. Polls with larger sample sizes are also weighted more than those with smaller sample sizes. The moving average is the weighted mean of these polls.
To obtain a range of likely values that each party could win if an election took place tomorrow, we sum two separate sources of error: sampling error and polling industry error.
Sampling error represents the risk that the views of a randomly chosen subset of the population do not match the views of the entire population. For each party, we estimate the range of values our moving average could have taken given the sampling error of each poll.
Polling industry error, or non-sampling error, represents the risk that all the polls are systematically biased in one direction or the other. Academic research has shown that election results fall outside the sampling error of polls more often than they would if sampling was the only source of error. Sources of polling industry error include using skewed samples, voters being undecided until election day, or voters not telling pollsters their true intentions. We estimate non-sampling error by considering how much the results of previous elections differed from their pre-election polling averages. The values we have chosen are based on a combination of academic and original research.
Please note that the majority of UK voting intention polls do not include Northern Ireland, where the major political parties are not the same as in England, Scotland and Wales. As a result, voting intentions in Northern Ireland are for the most part excluded from our poll tracking.
With thanks to Jack Bailey of the University of Manchester for his help with polling aggregation methods.
