Skewed sampling

Some pundits predicted general election success for the Tories on the basis of their local election performance. Not so fast
May 19, 2006

At the local elections on 4th May, about half of the total electorate had a vote. The Tories ended up with a net gain of around 320 seats, and so it is tempting to assume that had there been polls for every local authority, the party would have picked up over 600.

But the picture is more complicated than that. The elections were only held in England—and in just over half of all authority areas there. Most important, those areas where voting took place make up a very skewed sample. Almost 1,800 contests took place in the 32 London boroughs, where every seat was up for grabs. Virtually all the metropolitan boroughs and other unitary bodies held elections and the 56 of these accounted for just over 1,100 seats. But in these places, only one third of seats were up for election. This was also the case with the 88 district councils where polling took place, but these made up only just under 40 per cent of all districts: the rest vote in other years.

The Tories made just about half their net gains in London. But London was heavily over-represented on the battleground. It accounted for 18 per cent of authorities which were up for election (against 10 per cent nationally) and had three times as many seats up for contention as compared with other places: an "oversampling" of 5.4 times.

The metropolitan and other unitary councils made up 32 per cent of the areas with a poll, but this compares with a 17 per cent share across England. So they were "oversampled" by 88 per cent. However, Tories made only nine net gains in these 1,100 seats.

On the other hand, only just under 40 per cent of district councils had elections this year, so they were "under-represented" by around 60 per cent. In this grouping the Tories picked up 140 seats.

Transforming this to a national (English) result, the Tories' gains become the equivalent of 29 in London, 20 in the metropolitan and unitaries, and about 360 in the districts—a total of 410 across the whole of England—rather less than 600.

Of course, at a general election it is seats rather than votes that settle the outcome. To overcome their perennial poor showing in Scotland and Wales, Cameron's Conservatives would have to make net gains of around 130 seats in England to command an overall majority. That is an increase of 70 per cent on present numbers. There is a lot of further ground still to be made up if that target is to be hit.