This page gives an updated version of the pre-election esimates of voter migration. The idea is to analyse voters by how each one voted in both 2010 and 2015. Many voters will have continued to support the same party, and a large number of voters switched allegiance. The earlier analysis was flawed because the opinion pollsters had the wrong party support figures. This version corrects for that error and uses the actual 2015 election result support figures.
The graphic shows how people switched:
The graphic shows the various migrations of one hundred typical voters from 2010 to now. Voters who have switched from one party to another are shown moving along the corresponding arrow. "Lost" supporters are shown in grey, and "gained" supporters carry a white plus sign.
There are four key changes: the collapse of the Liberal Democrats; the rise of UKIP; the SNP surge in Scotland; and the growth of the Greens. On the graphic, we see five outbound arrows from the Lib Dems, and several inbound arrows into the three insurgents.
Compared with the pre-election estimates, there are the following differences:
These are direct transitions from 2010 voting choice to 2015 voting. For example, the two voters moving from Lib Dem to UKIP represent the fact that two per cent of the GB electorate chose to vote Lib Dem in 2010 and then switched their votes to UKIP in 2015. In other words, less than one tenth of 2010 Lib Dem supporters defected to UKIP in 2015 (two out of twenty-four).
The raw data for this article comes from looking at the detailed tables of four YouGov surveys from 10-17 April 2015 (Sunday Times, Sun, Sun, Sun) with a total sample size of 7,938. Combining these polls together gives us a table of current voting intention against how people voted in 2010:
These raw data are then adjusted and extended to make a "transition matrix" for the six major parties and "Other". Each column of the transition matrix should sum to 100, and shows where one hundred supporters of that party in 2010 have migrated to. Most voters stay with their original party (for example, 82 out of 100 Conservatives have stuck), but many Lib Dem voters have switched to other parties leaving only 28 voters out of their original 100.
The adjustments made are the minimum necessary to make the matrix consistent with the measured levels of support in both 2010 and the actual 2015 vote share:
In particular, the matrix product of the transition matrix with the 2010 vote share should be equal to the 2015 vote share. For instance,
Some assumptions about the minor parties had to be made to do this. These were:
Finally, the transition matrix can be scaled by the 2010 vote share to get the number of voters in each category:
(Note that not every row and column exactly adds up due to rounding.) On the actual graphic, there were some additional simplifications: