Projecting The 2001 Election Onto Current Constituencies

In my last post I noted that to analyse opinion polls, I would need to project the June 2001 general election results onto the current constituencies. To look at a method for this, I will use a nearby seat, Romsey & Southampton North.

The Media Guide To The New Constituencies by Colin Rallings and Michael Thrasher gives us some useful information. Firstly, if this constituency had been in existence at the May 2005 general election, the three main parties would get:

• Liberal Democrats - 19,217
• Conservatives - 19,013
• Labour - 4,816

It also goes on to tell us that it drew from 3 constituencies:

• 56,979 of Romsey's 70,632 voters (the remainder going to Winchester
• 5,137 of Southampton Test's 74,401 voters (the remainder remaining with Southampton Test
• 3,345 of Southampton Itchen's 77,241 voters (the remainder either remaining with Southampton Itchen or moving to Southampton Test)

Take Romsey first, and compare the 2001 results with the 2005.

We find that, in 2001:

• The Liberal Democrats got 291 more votes than in 2005
• The Conservatives got 1,954 fewer votes than in 2005
• Labour got 444 fewer votes than in 2005

We can then multiply these by 54,979 and divide by 70,632 to get an approximation for how many of these are in the area that becomes Romsey & Southampton North:

• Liberal Democrat - 227 more
• Conservative - 1,521 fewer
• Labour - 346 fewer

Next, we compare Southampton Test in 2001 with 2005, and find that in 2001:

• Labour got 3,979 more votes than in 2005
• The Conservatives got 210 fewer votes than in 2005
• The Liberal Democrats got 2,846 fewer votes than in 2005

So, we multiply these by 5,137 and divide by 74,401 to get the result for the parts of Bassett and Swaythling that got transferred over:

• Labour - 275 more
• Conservative - 14 fewer
• Liberal Democrat - 197 fewer

And finally, we compare Southampton Itchen in 2001 with 2005, and we find that in 2001:

• Labour got 1,682 more votes than in 2005
• The Conservatives got 239 fewer votes than in 2005
• The Liberal Democrats got 2,967 fewer votes than in 2005

Hence, we multiply these by 3,345 and divide by 77,241 to get the results for the part of Swaythling that got transferred over:

• Labour - 73 more
• Conservative - 10 fewer
• Liberal Democrat - 128 fewer

Putting these all together, we get the 2001 result for Romsey & Southampton North:

• Liberal Democrat: 19,217 + 227 - 197 - 128 = 19,119
• Conservative: 19,013 - 1,521 - 14 - 10 = 17,468
• Labour: 4,816 - 346 + 275 + 73 = 4,818

Now we look at the actual result for Romsey & Southampton North at the May 2010 election:

• Conservatives - 24,345 (up 6,877 from 2001)
• Liberal Democrats - 20,189 (up 1,070 from 2001)
• Labour - 3,116 (down 1,702 from 2001)

This is quite a simple seat, really. The Labour vote goes down, and the Conservative and Liberal Democrat vote goes up (by a total of 7,947. We assume that the disappearing Labour vote has gone to the 2 Government parties and pro rata this, so that 1,473 switched from Labour to the Conservatives between 2001 and 2010, and 229 from Labour to the Liberal Democrats.