Key marginals: Methodology

How were the seats chosen?

We selected 49 from the 100 most marginal seats in Britain.

Our aim is to provide examples of seats from each of the nations and regions where the outcome of the 2015 general election is likely to be decided. Our selection was designed to illustrate some of the different characteristics of such seats.

We have included examples with very small majorities and others with larger ones.

We have included seats that are primarily contests between Conservative and Labour, between Conservative and Lib Dem and between Labour and Lib Dem.

We have also included seats held by the Scottish National Party and Green Party.

The selection is not designed to be mathematically representative, but to give interested readers a flavour of individual contests across the 2015 political battleground.

How was the data chosen?

Three key indicators have been selected to illustrate each constituency's socio-economic profile.

Income is measured using the median annual gross income statistic from the Office for National Statistics' Annual Survey of Hours and Earnings .

The +/- percentage figure is the "confidence interval". This means that the ONS is 95% sure that earnings are no more than this percentage higher or lower.

Unemployment is illustrated by the ONS claimant count - the percentage of people claiming unemployment-related benefits that month.

The BBC tracks unemployment data for the whole country here.

Ethnicity data is derived from the 2011 Census for England and Wales, and Scotland.