What effect did MORI's wording have?
“In the next election for Mayor of London, the present Mayor, Ken Livingstone is standing for re-election as the Labour Party candidate. Boris Johnson is standing for the Conservatives, Brian Paddick is standing for the Liberal Democrats, Siân Berry is standing for the Green Party, and there will be other candidates too. In the election for Mayor, voters will have two votes, one for their first choice as Mayor and one for their second choice. If the election were held tomorrow, which candidate would be your first choice?”
A couple of people have raised questions in my comments and over on Mike Smithson's site about the wording MORI used in today's mayoral poll - reproduced above. People raised two points, firstly, could the extra mention of Ken Livingstone being the incumbent have helped him and, secondly, what might the effect be of prompting people with Sian Berry's name?
In theory reminding people of Livingstone's incumbency could help him (or indeed hinder him!), but I suspect that in practice the effect wouldn't be large - most people likely to vote probably know Ken is the mayor. Obviously we can't actually tell without trying parallel questions with alternate wordings, but I find it unlikely. Certainly it can't explain any of the difference between this and the YouGov polls, since YouGov also preface their question with text saying that the post is currently held by Ken Livingstone.
More interesting is the effect of including Sian Berry amongst the names in the prompt. In contrast to MORI's wording, in YouGov's question respondents were shown only the names of Livingstone, Johnson and Paddick - they were only given the names of the other candidates after opting for "another candidate". This MORI poll found Sian Berry at 5%, the highest that any of the minor party candidates has managed in any of the polls so far. Was that because of the prompting?
In this case we do have a very direct example of what effect including the Green party in the prompt has. In last year's elections to the Scottish Parliament YouGov prompted only with the Conservatives, Labour, Lib Dem and SNP. A few weeks before the election the Green's were showing only 4% in YouGov's polls. In fact, overall the figure for the minor parties looked incredibly low compared to the result at the previous Scottish election. As a result Peter Kellner decided to change YouGov's question wording halfway through the campaign to include the Greens and other minor parties in the prompt. The result was that in the next YouGov poll a week later, the level of Green support jumped up to 9%. In other words, including the Greens in the prompt more than doubled their support.
If we know including minor parties increases their support in polls, that leaves the question of whether it produces more accurate answers? In the case of YouGov's Scottish polls it probably didn't - their final poll had the Greens on 9% when the Green party ended up getting only 4%, so they'd probably have been better off leaving them out of the prompt. On the other hand, polls that don't prompt at all tend to underestimate the level of Lib Dem support. Unfair though it seems, the right approach seems to be to prompt by the name of the big three parties, but not all the minor parties.
In the case of this particular poll, the decision to include Sian Berry in the prompt, but not the other minor parties, seems very strange given that the Green party finished behind UKIP, Respect and the BNP at the last election.
