The major UK pollsters consistently record a variance in support for the Liberal Democrats. There are two possible reasons for this - what question is asked and what weighting is made on the results. Some pollsters offer a party list, some rotate the order, some add in 'Don't know' all offer others. It is well established that the Lib Dem vote usually rises in the last few days of an election campaign as the Party gains the pledges of a higher proportion of 'floating' voters than it does of hard core "I always vote X and always will" voters. It is not surprising then that the Party fairs less well when there is a 'Don't know' option. YouGov uniquely offers 'Don't know' and also offers 'won't vote'. The Lib Dems best performance is with ICM which does not give either of these options.
The actual questions asked are as follows:
IpsosMori: How would you vote if there were a General Election tomorrow? Conservative/Labour/Liberal Democrat/Other
ICM: If there were to be a general election tomorrow which party do you think you would vote for? Conservative/Labour/Liberal Democrat/Other
Populus: If the general election was tomorrow, which party would you vote for? Would it be [rotate order] Conservative, Labour, Liberal Democrat, or another party - or would you not vote at all?
YouGov: If there were a general election tomorrow, which party would you vote for? Conservative, Labour, Liberal Democrat, Scottish Nationalist/Plaid Cymru, some other party, would not vote, don’t know.
ComRes: If there were a general election tomorrow, would you vote Conservative, Labour, Liberal Democrat or some other party?
Populus: If the general election was tomorrow, which party would you vote for? Would it be [rotate order] Conservative, Labour, Liberal Democrat, or another party - or would you not vote at all?
YouGov: If there were a general election tomorrow, which party would you vote for? Conservative, Labour, Liberal Democrat, Scottish Nationalist/Plaid Cymru, some other party, would not vote, don’t know.
ComRes: If there were a general election tomorrow, would you vote Conservative, Labour, Liberal Democrat or some other party?
The second issue is what is done with the raw results - some pollsters amend numbers based on 'certainty to vote'. ICM and Populus apply a weighting based on previous regularity of voting. IpsosMori does not. This could provide a bias to Labour in Mori polls as it is well demonstrated in recent general elections that the actual Labour vote turned out to be lower than polled vote and reflects the old adage that 'A high turnout is good for Labour'. For more on this I defer to Anthony Wells post in his excellent UK Polling report to which I am indebted for much of the data on this subject.
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