Driven by data; ridden with liberty.
On May 4th 2017, the British public will be voting in local elections across England, Scotland and Wales. Elections will be held in 34 councils in England, all 32 councils in Scotland, and all 22 councils in Wales. There will also be elections for six new Combined Authority mayors, and two elections for local authority mayors. The Manchester Gorton by-election, held after the passing of Sir Gerald Kaufman, will also be contested on this day.
In each year, only a selection of councils have some, or all, of their seats up for election. The type of councils, and their political composition, varies from year to year. To judge performance, it is insufficient to merely look at the total number of votes cast for a particular party, or the percentage of councillors won.
The question arises: how do we effectively measure local election performance, to account for any variation in councils?
To answer this question, two teams independently two similar measures.
If you watched the BBC’s coverage, you will likely hear about the ‘projected national share of vote’ (or PNS). This measure is calculated for the BBC by a team led by Professor John Curtice and Professor Stephen Fisher.
Using a sub-sample of council wards, the PNS is an estimate of what share the principal parties would have won under the following assumptions:
The National Equivalent Vote share (NEV) is calculated by Professor Colin Rallings and Professor Michael Thrasher, both of the University of Plymouth. The NEV is produced each year for the Sunday Times.
Both of these measures purport to show the same thing, but due to methodological differences, there is small and limited disagreement between them.
A basic fact about these two measures seemingly requires reiteration: these are measures of local election performance, and are not equivalent to general elections.
Local elections are for local councillors. General elections are for Members of Parliament. Even when local elections and general elections are held on the same day, the results are not equal.
This phenomenon — where voters vote differently in different elections on the same day — is called ‘split-ticket voting’. It is reasonable to expect voters to behave differently in different elections, varying from year to year. The University of Plymouth’s Election Centre presented an analysis into split-tickets in British elections in 2015.
Local election vote shares, under the PNS or the NEV, do not — in any fashion — ‘disprove’ general election polling.
How the major parties perform under these local election measures remains to be seen.