Percentage changes of notional constituencies

[Harry Hayfield]

I would like to know if someone could explain (when constituencies change) we seem to get a large number of percentages that do not on the face of it make any sense. At the 2010 general election, Aberconwy was one of those seats and on the Press Association's notional calculations website it announced "45.40% boundary change", whereas on UK-Elect it says "Based On: 15.9% from Meirionnydd Nant Conwy, 84.1% from Conwy, Which Is: 20.7% of Meirionnydd Nant Conwy, 67.3% of Conwy. Total Change % (Conwy to Aberconwy): 40.30%"

Therefore could someone explain what those percentages mean and how I read them with regard to the past constituencies? "For instance, how much of Aberconwy was in Conwy and how much of it was in Meirionnydd?"

[lennon]

I have no idea how they calculate the "% Change" numbers, but to answer your last question:

Of the new Aberconwy seat - 15.9% used to be in Meirionnydd, and 84.1% used to be in Conwy

Of the old Meirionnydd seat, 20.7% was moved into the new Aberconwy seat
Of the old Conwy seat, 67.3% was moved into the new Aberconwy seat

The reason for the difference between old and new is obviously the differential sizes of the old and new seats.

[ajthomson]

The index of change is: I = (r + a) / E * 100

where
I = index of change
E = electorate of base constituency (by definition the largest contributor to the new seat)
r = electors removed from base constituency
a = electors added from all other constituencies

I've always found the index figure in absolute terms to be meaningless, but in comparative terms it's quite useful. Seats where the index of change is over 100 are ones where more than half the new seat wasn't in the old seat (I think, but my maths skills are a bit rusty).

[lennon]

Apr 4, 2016 17:19:29 GMT ajthomson said:

The index of change is: I = (r + a) / E * 100

where
I = index of change
E = electorate of base constituency (by definition the largest contributor to the new seat)
r = electors removed from base constituency
a = electors added from all other constituencies

I've always found the index figure in absolute terms to be meaningless, but in comparative terms it's quite useful. Seats where the index of change is over 100 are ones where more than half the new seat wasn't in the old seat (I think, but my maths skills are a bit rusty).

That makes sense - using the % numbers Harry gave above:
Take the Electorate of the new Aberconwy to be 100.
Then 15.9 have come from Meir, and 84.1 from Conwy.
The Total for the old Conwy is 124.96 (calculated as 84.1 / 67.3%) and so 40.86 have moved out.

The Index of Change is thus = (40.86 + 15.9) / 124.96 = 45.42% (expect slight difference due to rounding)