Sunday, April 18, 2010

Why Lib Dems don't do as well as other parties

My post on election campaigns not mattering now seems rather quaint, with the Lib Dems surging in the polls. Of course this is not completely novel, the idea of a 3rd party 'surge' was enough ingrained in popular conciousness for Spitting Image to joke about David Steel 'feeling' it.

Anyway what is perhaps becoming more clear is that even if the Lib Dems have the same percent of votes as Labour and the Conservatives, they do much worse in terms of numbers of seats. Most of the calculators put a 30% each election as something like Labour on 300 seats, Tories on 200 seats and Lib Dems on 100 seats.

I explained this on Tim Worstall's site by:

Itís not so much the disposition of the seats but the FPTP system itself, which simply favours geographically concentrated votes. So say the Tories take 60 percent in all southern seats, Libs 30 percent and Lab 10 percent, and the reverse is true in the north. Nationally if equal no. of seats in north and south then vote share is Con 35, Lab 35, Libs 30. But Libs have no seats.

I wasn't entirely sure if this was right, but I think it is essentially correct - the Lib Dems' support is too widely spread, and they do reasonably well in most seats, not especially well in enough. Here is a chart of each party's % share of the vote in the 2005 election, starting with each's seat where they got the highest share of the vote in % terms, and ending with their lowest. So the 1st point on the chart is not a particular seat, but for each party the share of the vote they receivedin the seat where they received their highest share of the vote.

The box shows the share of the vote - 40% and higher - that typically wins you a seat. Of the 614 seats won by one of the three main parties, only 45 were won with less than 40% of the vote. Similarly in only 32 seats did a party get more than 40% and NOT win.

So taking the 40% line, one can see the Lib Dims get about 60 seats, the Tories 200 and Labour 350 or so - about what happened.

Now let's assume the vote share - 35.3 Labour, 32.3 Tory and 22.1 Lib Dem in 2005 - becomes 29.9 Labour, 29.9 Tory and 29.9 Lib Dem - not wildly dissimilar to some recent polls.

Now the first thing to note is that the % share of the vote when a candidate will typically win will fall, to something like 37%. This is simply because the Labour and Conservative share has fallen, and we are in a three-way tussle.

But again we can see a good estimate of how many seats each party will get from where their line crosses that 37% line - the Lib Dems about 150, the Conservatives about 210 and Labour about 280. Now this isn't quite what the uniform polls predict, that is because of a variety factors such as boundary changes since 2005, the particular makeup of some seats whereby Labour can win with a slightly smaller share than the Libs and so on. But it does show us why the Lib Dems fail to match Labour. And basically it's because their vote is spread reasonably evenly, with still high % shares of the vote in the last 200 constituencies, whereas the Tory and Labour vote has collapsed.

This assumes a uniform national swing (UNS), so the Lib Dems have gained 8% of the vote nationally since 2005 and will gain 8% in every constituency. What they need to form a government is for that extra 8% nationally to be concentrated in seats 200-400, where it would win them the election.