Uniform national swing is one of those psephographical tricks that shouldn’t work, but does. There’s not much reason to believe that the local swing towards a party will reflect the national swing towards that party.
Yet UNS has a tremendous record. Suppose that we take two adjacent elections, and `de-mean’ each constituency result — that is, subtract the national average.
If uniform national swing is right, then these de-meaned constituency results should be very closely related. (Contrariwise, if the demeaned constituency results were very different, that would mean that lots of local swings had interfered with the pattern seen in the previous election).
That’s exactly what we find. Taking Danny Dorling’s historical archive of election results, then the pairwise correlation of demeaned constituency results for twentieth century elections for the Conservative party is astonishingly high — between 0.97 and 0.99. You ordinarily don’t get correlations of that magnitude in social science.
But in 2015, UNS is going to face severe problems because of the collapse of the Liberal Democrats. In 2010, the Lib Dems polled 23% of the vote, six percentage points behind Labour. Currently, the UK Polling Report average has them on 10%, implying a swing of thirteen percent away from the party.
The problem is that in seventy one constituencies across the country, the LibDems had a vote share of less than 13%. UNS therefore implies negative vote shares.
You might say that this doesn’t particularly matter, since UNS is usually used for predicting seat shares, and the Lib Dems were never going to win these seats in any event. But it does still matter, because in these seats other parties cannot gain as much from the Liberal Democrats as they do averaged across the country.
Now, the LibDems might recover in the polls — and it’s hard to see them falling much further. If they recover to 14.3%, as Steve Fisher suggests, then UNS will only be `broken’ (in the sense of predicting negative vote shares) in the eleven constituencies where the LibDems polled less than 8.7%. But the 2015 election will nonetheless prove a more difficult test for UNS than some recent elections.