|Hung Parliament. Labour short by 17|
The standard method of translating shares of the vote into seats is to use a uniform swing calculation. This means that the national change in vote share for each party is applied to each individual seat to see how that would effect the result, and then these theoretical results in each seat are totted up to produce a projection for House of Commons.
This is a crude measure and can result in some illogical and impossible projections – for example, if a poll showed Labour support dropping by 13%, as one poll did during Summer 2008, then a uniform swing calculation using those figures would project Labour getting less than zero votes in 48 seats. This is clearly nonsense. Such projections also ignore any regional variations, tactical considerations or variations due to incumbency effects of new MPs or MPs standing down. Despite all these drawbacks, it normally does a reasonably good job and, given that it is a straight extrapolation of current voting figures it is at least accepted as a fair projection that is not at the whim of individual guesswork or assumptions.
UPDATE: Even with all the caveats above, the rise of the SNP in Scotland meant that a plain UNS projection was starting to look really quite absurd. From February 2015 I’ve moved slightly from straight UNS – instead there is UNS in Scotland, and UNS in the rest of Great Britain. The Scottish UNS is based on an average of recent Scottish polls, the figures for the rest of the country are based on the average of GB polls, but adjusted to account for Scotland.
For the avoidance of doubt, this is NOT my prediction of what will happen at the election. It is a uniform swing projection based on the current average of the polls – it doesn’t take account of how the polls might change before may, or things like incumbency, by-elections or constituency polling. It is not what I expect to actually happen in May.