Marginal improvements. Looking at the reliability of seat predictions from polls

# Marginal improvements. Looking at the reliability of seat predictions from polls

Last week, Mike Smithson noted the Conservatives seem to have an in-built advantage in the electoral system over Labour – if they got an equal number of votes, the Conservatives could expect about 15 seats more than Labour even if Labour had a 0.5% lead in the polls, if Electoral Calculus is to be believed. That begs the question whether seats are likely to move consistently at the next election in the way that seat predictors assume. Let’s have a look at the possibilities.

Electoral Calculus uses what is called a strong transition model. For major parties in seats in which they are in contention, this is close to uniform national swing. For present purposes, I’m going to stick with looking at uniform national swing, which has the great merit of simplicity.

First things first, what is uniform national swing? It assumes that as votes move from one party to another, they will do so in precisely the same percentages in every constituency. So if the Conservatives hold a seat with 51% of the vote and Labour have 46% of the vote, a 5% swing would see those percentages reversed. If UNS applies, then if the Conservatives had just 20% in a seat and Labour had 70%, we should expect the Conservatives to drop to 15% and Labour to get 75%. The point to note is that you look at the absolute percentages, you don’t try to adjust for the proportion of voters in each constituency.

Logically, UNS can’t predict seat counts in all contingencies – it breaks down completely mathematically in more extreme cases (this is what Electoral Calculus’s adjustments for a strong transition model are designed to overcome). Obviously, seats do vary according to local circumstances and the quality of candidates on offer, and few seats will swing by exactly the national swing (Mike’s home seat, Bedford, is quite unusual in that it has tracked the national swing closely at every election since its creation in its modern form in 1997). In practice, UNS has done pretty well in normal circumstances in coming up with broadbrush predictions, particularly in two party systems.

Why? Two different points are worth considering. First, when you’re dealing with a large number of swing seats, you’re likely to produce a bell curve of distributions of swing, clustered particularly around the median swing. All other things being equal if the swing isn’t outlandishly big, those large numbers of seats are likely to be almost randomly distributed by size of majority across the bell curve. A seat might fall to an unexpectedly big swing but it is likely to be counterbalanced by a seat that didn’t fall owing to an unexpectedly small swing.

Secondly, seats of a similar type are likely to behave similarly (and probably did so in the past too, assuming they haven’t changed). Lincoln, Bolton West and Keighley, for example, have oscillated between the two main parties for decades. So the ordering of seats at a previous election is likely to be quite indicative of the ordering of seats at the next election, though the colour of the rosette of the winner might change.

Do these considerations hold next time? Well, the second one looks very suspect. Here are the marginals that the Conservatives are defending and here are the marginals that Labour are defending (including all seats vulnerable to a 10% swing). Focus on the final column, which shows you what the swing was last year. Nationally, the swing was about 2.2% from the Conservatives to Labour.

But when you look at the most marginal seats, the swings vary wildly and no pattern is easy to discern. Stroud and Bishop Auckland nestle side by side in the Labour defence list at numbers 12 and 13, but Stroud secured an outsized swing to Labour while Bishop Auckland saw a solid swing to the Conservatives. On the Conservative defence list at numbers 12 and 13, Broxtowe and Stoke-on-Trent South show similar and similarly contrasting swings.

The last election saw relatively few seats change hands but both main parties’ coalitions seem to have changed substantially, whether because of Brexit or because of the very different direction that Jeremy Corbyn has led Labour. It seems unlikely that seats that responded very differently to those stimuli will respond similarly to either the continuation or the withdrawal of those stimuli. Broxtowe and Stroud might swing in similar ways next time, and Bishop Auckland and Stoke-on-Trent South might swing in similar ways next time, but all four swinging in harmony does not look particularly likely.

There is a betting consideration to this. Next time the constituency markets will not be proxies for the national result. You won’t be able to deduce from a 3% swing in the polls from the Conservatives to Labour that Chingford & Woodford Green, North East Derbyshire and Carlisle all look in more or less the same amount of peril for the blue team. Constituencies will need to be considered either individually or in relatively small groups.

But what of the bell curve? I’ve had a look at the 2017 results and plotted the seats by size of swing, grouped into bands, here. As you can see, this looks pretty much like a bell curve centred around the 2.2% swing to Labour I mentioned earlier. Not only that, the seats in each segment divide roughly evenly between the blue and the red team in each band (with the exception of those seats which swung up to 5% to the Conservatives, which are twice as likely to be Conservative-held as Labour-held).

So UNS, and similar methods, look as though they still have life in them. What you shouldn’t do next time is look at a given seat and extrapolate even indicatively how it is likely to perform next time from an opinion poll. Local considerations look set to be far more important than national trends when looking at individual seats.