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Going back to your constituencies. Alastair Meeks on not taking seat predictors too seriously in times of change

June 5th, 2019

Obviously, you should not treat opinion polls very seriously at all, especially when no general election is on hand. Respondents are being asked an artificial question (there is no general election tomorrow) with no real-world consequences hanging on their answer. 

You might be getting a general expression of enthusiasm for a particular credo, a message to the voter’s normal choice or simply a casual choice made without much thought, and that’s before you get into the methodological adjustments that the pollsters use to produce their confection. Headline voting polls at this stage in an electoral cycle should be treated as an expression of mood rather than a forecast.

For all that, they are pretty well the only useful data points that we have for Westminster elections. So they are pored over endlessly by political geeks, deconstructed by subsamples and slotted into seat predictors.

The latest YouGov poll was particularly sensational, putting Labour and the Conservatives in joint third on 19% behind the Lib Dems on 24% and the Brexit party on 22%. Not content with this, Nigel Farage complained that YouGov’s failure to prompt for the Brexit party nefariously suppressed his creation’s ratings (regrettably, he did not give a detailed psephological explanation of how YouGov should correct for their past overstatement of Brexit party support: no doubt they would have welcomed his new-found expertise).

These findings were then plugged into the seat predictors. What was most striking about these was how Labour in particular, but also the Conservatives, held onto far more seats than one might expect. Electoral Calculus comes up with Labour 202, Brexit 141, Lib Dems 119, Conservatives 110.

Forgive me if I have my doubts about the accuracy of such predictions. Seat predictors are made for a world where polls move up or down a few percentage points, like boats bobbing on the swell of the ocean. If this poll were to be anything like accurate, Labour and the Conservatives are about to be hit by a rogue wave.

The traditional method of calculating seat movements at elections is by uniform national swing. That implies that the percentage swing between parties is exactly the same in every seat. So if the Conservatives start on 48% in a seat and Labour on 30%, and there’s a 10% national swing from the Conservatives to Labour, Labour would be expected, all things being equal, to take the seat 40% to 38%. Historically it has worked better than the more intuitive proportional swing (where each parties’ share of the vote would move proportionately in each seat).

The problem with uniform national swing where the movements are so wild is that it breaks down completely. If, for example, the Conservatives were to drop from 43% to 19% at successive elections, then uniform national swing would imply that the Conservatives got negative votes in every seat in which they tallied 24% or less of the vote in 2017. There were 98 such constituencies; this is not a theoretical problem.

Electoral Calculus is more sophisticated than this. It operates what is known as a strong transition model. As the link explains, the model divides party supporters into strong and weak supporters. The model allocates them to each seat based on vote share in each seat, and then assumes that the weak supporters fall away first when a party’s polling droops.

There are two difficulties with this approach in turbulent times, particularly so far as both Labour and the Conservatives are concerned. First, it assumes that their votes would drop off particularly quickly in seats where they start from the position of a low vote share. This might well be true for minor parties or parties with particular regional strength.

But both main parties have substantial and to date enduring strength across the nation: Labour did not lose a deposit in 2017. It is not at all obvious that beyond a certain point in any given constituency that their voters are weak.

Secondly, and more fundamentally, the model copes poorly with a fundamental realignment. Many lifelong former Labour supporters voted for other parties in the EU elections. An absolute majority of Conservative party members appear to have voted for the Brexit party in the EU elections. The concept of a strong voter breaks down when new binary divides spring up.

None of this is to disparage electoral models and Electoral Calculus is excellent. They have proved terrifically useful in the past in understanding the dynamics of close battles (and you should always remember that the predictions are only ever going to be as good as the inputted data). But where the public shifts allegiance dramatically, they are going to be much less useful. It would be like predicting the outcome of a duck race in a whirlpool.

At the vote shares indicated by that sensational YouGov poll, my expectation is that both Labour and the Conservatives would have far more wasted votes than any electoral model currently suggests and as a result lose considerably more seats than any model put forward.

We’ve since had an Opinium poll showing the Brexit party in the lead and the Lib Dems in fourth. Electoral Calculus showed the Brexit party with 306 seats and Labour with 202, but with the Conservatives on a mere 26 seats.  I have to say that feels a more believable outcome from such a result.

In summary, supporters of the main parties, particularly the Conservatives, should not assume that incumbency and traditional strength is going to save them if the polling continues to show them gurgling round the plughole, even if the seat calculators suggest otherwise. This is no time for complacency.

Alastair Meeks