Marginal polls

Back in May when ComRes first launched their marginal seat Omnibus I wrote about some of my reticence towards marginal polling, why it isn’t usually quite as useful as it should be, and why I hoped that might change. Marginal seat polls matter because they are the seats that might change hands, and therefore the seats that will decide the election. If they behave differently to the national polls, and if different groups of marginal seats behave differently to one another, it’s obviously a very big deal.

What has limited their usefulness in the past is their infrequency, and the lack of comparability and empirical testing. Marginal polls used to only come along occasionally, varied a lot, polled different groups of seats, and didn’t often happen right before elections so weren’t tested against reality, meaning methods weren’t finessed and improved over time in the same way national polls are.

In practice their rarity and inconsistency rendered them a very blunt tool when we’re looking to spot quite subtle differences – the reality is that marginal seats aren’t that different from the country as a whole:

  • In English & Welsh seats at the last election (the swing in Scottish seats is consistently different) the average swing from Lab to Con was 5.8%. In the 50 most marginal seats the swing was 5.6% – no real difference at all. In the real core battleground (Lab-v-Con seats), there was a slightly more noticeable difference, but it was still small. Amongst all Lab-v-Con seats the swing was 6.7%, amongst those with a majority of less than 10% the swing was 8% – so 1.3 percentage points bigger.
  • In 2005 the average swing in all English seats was 3.2%. In the Lab-v-Con battleground seats it was 3.5%, in Lab-v-Con marginal seats the swing was also 3.5%. No difference.
  • In 2001 the average swing in all English seats was 1.6%, the average swing in Lab-v-Con seats was also 1.6%, the average swing in marginal Lab-v-Con seats was -0.5% (that is, overall there was a small swing to the Conservatives, but on average there was a tiny swing to Labour in the Lab-v-Con marginals).

You can see that marginals do behave a little differently sometimes – the Conservatives managed a better swing in their target Labour marginals in 2010, Labour did better in those seats where they had fresh incumbency in 2001 – but the differences aren’t huge. We’re talking 1 or 2 percent difference. That’s enough to make a genuine difference in seat numbers, but is very difficult to determine from a single opinion poll. The difference between the national picture and the marginal picture will normally be so subtle that it could easily be lost under or mistaken for normal sample variation, or the methodological differences in doing marginal polls (or vice-versa, normal volatility or methodological impacts could be mistaken for a different pattern in the marginals when there is none).

More recently though things have been looking up. We’ve seen an increase in marginal polls and, more importantly, we’ve seen an increase in regular marginal polls – Lord Ashcroft and ComRes are both doing regular polls of the same groups or group of marginal seats. Different pollsters are also doing marginal polls of roughly the same marginal seats – Ashcroft, ComRes and this week Survation have all done polls that include ultra-marginal Conservative -v- Labour seats. However, despite covering the same ground, the results are very different.

The table below is an attempt to make the results roughly comparable. There are much more obvious differences between different battlegrounds (that is, between seats that are Con-v-Lab battles and seats that are Con-v-LD battles), so I’ve looked at only the Con-v-Lab battleground – those marginal seats with the Conservatives in first place ahead of Labour. Sample size for each poll is just for the Con-v-Lab marginals, the swing just those seats, and I’ve compared it to the average of national polls at the time of each marginals poll’s fieldwork.


As you can see – three companies, three completely different stories. ComRes show the Conservatives doing much better than nationally in these key marginals. Lord Ashcroft shows very little difference between the national picture and equivalent marginals. The Survation poll today showed Labour doing much better in similar marginals.

Some of the differences will be methodological. For example, Ashcroft uses the two stage question wording to try and coax out local considerations though frankly it makes very little difference in Con-v-Lab marginals. I don’t think ComRes prompt for UKIP in their marginal polls, but Survation and Ashcroft both do. The weighting regimes are very different – I think Ashcroft weights by age, gender, social class and recalled vote; ComRes weight by the same plus housing tenure; Survation appear to weight only by age and gender, with no political or socio-economic weights. Lord Ashcrofts poll are also, it’s worth noting, of a substantially larger size – they are an aggregate of full size single constituency polls, rather than a poll of a group of marginals.

You pays your money, you takes your choice. My own expectation is that, if there is a relatively small Con to Lab swing the Conservatives will do very slightly better in the marginals thanks to the double-incumbency effect – the historical evidence for such an effect is extremely strong and I see no obvious reason for it not to happen this time round. If, on the other hand, there is a hefty swing towards Labour then it might well be cancelled out due to a stronger performance in Labour target seats, like we saw for the Conservatives in 2010 or Labour in 1997. Time will tell. Either way, I wouldn’t expect Con-Lab margins to perform radically differently to the national picture – if there’s a systemic difference between marginals and the country as a whole, I’d expect it to be a small one. In a close election that could still be the difference between a majority and a hung Parliament, so don’t underestimate its potential importance, but it would be a remarkable election if the swing in marginal seats really was 4 or 5 points bigger or smaller than the national picture.

102 Responses to “Marginal polls”

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  1. @John Pilgrim
    “My comment related more to “incumbent” opposition candidates, in the sense of being selected for a substantial period or for successive campaigns.”
    Fair enough. What I wrote was not my view, but referred to Chatterclass’ comment “Incumbency: nothing magical in it. An MP who has been working the seat since the previous election, with a paid for constituency office and staff has a huge advantage against a candidate, probably only selected a year or two before an election and reliant on donations and volunteers.”
    That seems to me to be fairly likely, while yours may also be true.
    How many “incumbent” opposition candidates are there? It must take grit to continue election after election in the other party’s safe seat.

    Do you really think that “pre campaign substantive political activity of opposition candidacy and branch and party action” persuades people to change their minds and vote for your “incumbent” opposition candidate?
    Most of them won’t get to the end of sentences like that.

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  2. DAVE
    Just shows you shouldn’t believe everything you read, even on this distinguished blog.
    It’s mainly in reality a quite slow and deliberative process, including selection debates at branch and, decisively, at constituency party level, often between people who have known and worked together for years in local politics, sometimes much shorter and “meet the candidate” when parachuted in, or self-submitted.
    Most people do it for pleasure, a bit like posting here as a participation in an intellectually and morally engaging debate. Street campaigning is done with people you like, and meetings often have the same anticipation as a boxing match.
    Perpetual or repeat candidates are rightly treated as politicians. This is not, to my observation, not by any means as a result of actually or as having the prospect of getting elected – many know they never will. Opposition politics is itself for most not primarily a negative pursuit but the active engagement in the implementation of policies and beliefs that the candidate fundamentally supports.
    It’s not a chore. For many candidates opposition politics is better than golf or train spotting and done with people you get to be fond of and admire, including the voting public.

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