The pointlessness of uniform swing calculations

Lots of interesting details to digest in today’s mammoth YouGov marginals poll (including, ahem, the importance which voters give to whether or not a candidate is local – in brief, it’s very important), but one point I’ve not yet seen anyone make is how heavily it undermines the seat predictions for the Liberal Democrats which come from uniform nation swing calculations made using Electoral Calculus.

These are very popular calculations, most notably regularly topping blog postings over at Political Betting, but compare the YouGov figures for the number of Liberal Democrat seats from the marginals poll with those from the national voting figures in the YouGov polls conducted at the same time and you find that the marginals polls gives the Liberal Democrats nearly double the number of seats compared with the national swing calculations (44 compared with 24).

The national YouGov polling at the time was putting the party on 16%, so it is hardly a surprise that both figures are down on the party’s current number of seats and given the range of factors such as YouGov’s consistent lower polling for the Liberal Democrats, the long period of time to the next election, the usual rise in the Liberal Democrat poll rating during the general election campaign proper and indeed the fluctuations in the party’s YouGov ratings since (up to 20% in the latest), it would be unwise to read anything much into the overall figures.

But the difference between the marginals and national polling, when both were conducted by the same organisation at the same time, does give a good reason to shout “that’s a load of nonsense!” at your computer (or just tut quietly under your breath and put on a cup of tea; your choice) next time you see an Electoral Calculus projection used to predict the number of Liberal Democrat MPs.

UPDATE: As touched on in the comments, my use of the phrase “uniform swing” as short hand wasn’t ideal as Electoral Calculus uses a modified proportional swing model. However, Electoral Calculus still essentially does a blanket calculation across the whole country, albeit one in two parts by using a standard calculation to split party supporters into strong or weak depending on the party’s share of the vote in a seat, and then applying swing calculations to each part. It is the poor predictive ability of these blanket calculations that is at issue, whether they are flat uniform swing or more complicated modified proportional swing calculations.

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22 Comments

  • David Heigham 21st Sep '08 - 12:18pm

    Anyone who applies uniform swing calculations to third (or fourth) party seat numbers simply does not understand what they are doing. Uniform swings would never have given the numbers that they start from.

    But for rough indications of the Lab v. Con seat numbers they are not radically misleading.

  • Yes, but … ATEOTD we’re still seats down on the last GE.

  • Liberal Neil 21st Sep '08 - 1:25pm

    No – it’s a poll, not a prediction.

  • David Evans 21st Sep '08 - 5:17pm

    Uniform swing projections are virtually totally invalid, but polls and such projections do give a guide as to how tough or easy seats will be to hold or gain. If we are above 20% at the time of the next election, we will do well; but if we are below 15%, things will be tough. In either case we will know where we need to go and fight. Now if it’s between 15 and 20%, it will be much more difficult to chose!

  • Clegg's Candid Friend 21st Sep '08 - 6:52pm

    What’s particularly interesting about this poll is that, in order to investigate the influence of tactical voting, the survey asked two questions.

    The first was a standard one about voting intention. The second followed a prompt about the possibility of tactical voting and a request to think specifically about the situation in the particular constituency.

    The response to the first question would have indicated much larger losses by the Lib Dems. Interestingly, this is because the Tory vote drops when tactical voting is considered, as well as the Lib Dem vote rising – and this happens even in Tory/LD marginals. I take it this means that some people are not so motivated to vote Tory if they are reminded that the main opponent is Lib Dem rather than Labour.

    Actually, I think the most usable finding is that on the basis of this survey our net advance in Labour/Lib Dem contests would be nil (one gain from Labour, one loss to the Tories coming from third place). In contrast the Tories would come from third place to win four of the ten Labour-held Lib Dem targets that were surveyed.

    Surely the plan of diverting our resources to 50 Labour-held seats (in many of which the Tories were second last time) should be reconsidered in this light – and preferably quietly dropped.

  • Clegg's Candid Friend 21st Sep '08 - 7:48pm

    Martin

    Name-calling again?

  • CCF,
    I don’t think you’re trolling, I think you’re drawing false conclusions about the net gain/loss to the LibDem vote share from tactical voting.

    I’d like to know on what evidence you base your assumptions.

  • Clegg's Candid Friend 21st Sep '08 - 8:48pm

    Oranjepan

    If you can explain in more detail what you mean, I’ll be happy to try.

  • Paul Griffiths 21st Sep '08 - 10:03pm

    Clearly any strategy targetted at Labour seats must also include tactics for diverting the resurgent Conservative vote.

    I must admit to some surprise and the number of Tory wins from third place suggested by this poll. Anyone know how many Labour wins from third place there were in 1997?

  • CCF,
    I don’t think the Conservatives have the best people, or the best policies, and I don’t think they are the best party. Yet the way you greet this poll shows you think they are better than us (even if you’d rather it weren’t so). I mean, what exactly does this poll (or, for that matter, any poll) actually prove?

    I think your statement:

    “The response to the first question would have indicated much larger losses by the Lib Dems. Interestingly, this is because the Tory vote drops when tactical voting is considered, as well as the Lib Dem vote rising – and this happens even in Tory/LD marginals. I take it this means that some people are not so motivated to vote Tory if they are reminded that the main opponent is Lib Dem rather than Labour.”

    is dodgy.

    You might well be correct, but there is no way of knowing for sure. The probability of tactical voting is that it depends upon the balance of views, which can vary strongly from one area to another and may indeed reverse the othodox expectation in some circumstances. And it may be that the opposite is truer than not.

    So it is therefore also vital to remain sceptical about stated voting preference and whether this is a direct indication of support for one or other party. It is all too easy to forget that it only takes a second to change one’s mind – however strongly held the previous belief!

    The old truism about democracy that ‘the public doesn’t know what it thinks’ still holds water: it is up to each party to provide the necessary information so that we can make the best choice as a society – it is always unwise to make any rash generalisations.

    I’d like to know why you take the current poll snapshot as a given reflection of what the public thinks, rather than using it as a means to explain what each side needs to do better to spread our message in preparation for when the meaningful ballot comes along?

    I’d therefore also like to know how you have come to your conclusion of whether we recieve an overall net benefit or loss from tactical voting.

  • Clegg's Candid Friend 21st Sep '08 - 10:55pm

    Oranjepan

    Sorry, but I thought you might have something sensible to say. My mistake.

  • Clegg's Candid Friend 21st Sep '08 - 11:36pm

    Paul Griffiths:
    “I must admit to some surprise and the number of Tory wins from third place suggested by this poll. Anyone know how many Labour wins from third place there were in 1997?”

    I can’t answer that question, but I agree the poll figures concerning these Labour/Lib Dem marginals are very surprising.

    On the first question – the raw “voting intention” – Labour drops by 9 points since 2005 (in line with other polls), but the Tories are up by 19 points and we are down by 11 points (a much larger switch than other polls would suggest, with the exception of the recent MORI poll that put the Tories at 52 and us at 12).

    When the prompt about tactical voting is put and then the respondents are asked how they will vote taking constituency circumstances into account, the drop in the Labour vote remains unchanged, but there is a sizeable transfer from the Tories to us. But even taking this into account, we are down by 3 points since 2005, and the Tories are up by 12. That explains why the Tories are predicted to come through from third place in so many of these supposed Labour-held Lib Dem targets.

    If these figures are accurate (and we have to bear in mind that the total sample size was huge – nearly 35,000), it suggests (1) that as far as fundamental voting preference is concerned (i.e. before tactical considerations are taken into account) Lib Dem support has dropped a lot more steeply in these Labour/Lib Dem seats than in the country as a whole and (2) that those who are naturally inclined to vote Labour are no more likely to switch to another party if they are reminded that the challenger is a Liberal Democrat rather than a Tory.

    To my mind, both those factors argue against a strategy of diverting resources towards Labour-held target seats.

  • Clegg's Candid Friend 22nd Sep '08 - 10:24am

    “… it suggests (1) that as far as fundamental voting preference is concerned (i.e. before tactical considerations are taken into account) Lib Dem support has dropped a lot more steeply in these Labour/Lib Dem seats than in the country as a whole …”

    I suppose this may well be because we benefited from a strong Iraq factor in 2005, which is not such a prominent issue now.

  • CCF – You are making a number of false assumptions in arriving at your incorrect deduction that we would only gain 1 seat from Labour based on the findings of this poll .
    Leaving aside the fact the poll was conducted in July and a similar poll now would give rather better LibDem results ( and no doubt a poll after the Conservative conference different results again ) , the poll looked at 14 Lab/LibDem seats . The Labour to LibDem swing in these seats was 3% and then applied to the results from the last GE to forecast which seats would change hands .
    Hence Durham which requires a 3.7% swing is forecast to remain Labour . Whilst the average Lab/LibDem may or may not be 3% the swings in individual seats will vary above and below this . A UNS from Labour to LibDem of 3% will not predict which particular Labour seats the LibDems would gain as they will gain seats with a larger swing and not gain others where the swing required is lower see Manchester Withington at the last GE for example

  • Clegg's Candid Friend 22nd Sep '08 - 12:15pm

    “You are making a number of false assumptions in arriving at your incorrect deduction that we would only gain 1 seat from Labour based on the findings of this poll .”

    As a number of people seem confused about this, I’d better make clear that all the data I have mentioned relating to the levels of party support and all the projections of the numbers of seats that would change hands come straight out of the report. They are not my “deductions”, incorrect or otherwise. And of course I have not presented them as reliable predictions of what would happen if a general election were held today.

    As for Mark Senior’s other points, yes, the fieldwork was done nearly two months ago (actually it was in late July and early August). You can’t expect a poll of this size to be analysed and published instantly, and you can’t expect it to be repeated every month. But it would be foolish to ignore its findings for that reason.

    And of course there is bound to be an element of projection in the conclusions drawn from any poll that doesn’t include the whole population. And of course swings aren’t going to be uniform even within the groups of seats chosen for analysis. Obviously the poll is giving us information about average party support within these groups. But none of that should be used as an excuse for ignoring this information.

  • Clegg's Candid Friend 22nd Sep '08 - 3:39pm

    Incidentally, I’m amazed that Mark Pack seems to think Electoral Calculus uses a uniform swing model. It hasn’t used a uniform swing model for more than four years.

    And since last year the Electoral Calculus model has included an additional modification specifically to try to correct under-prediction of the number of Lib Dem seats.

    Not that I’m suggesting this is an adequate model, before someone suggests that. But it’s certainly not a uniform swing model.

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