The Press Association has a long and proud history of covering UK elections and has been a trusted source for elections data since the 19th Century. Over time it has established conventions for handling this data that differ in some respects from the approaches adopted by some other media organisations and elections pundits.
Customers should be aware of the differences as these can help explain apparent data discrepancies.
PA normally uses the most recent state of representation at the time of an election as its baseline for seat changes rather than the state at the outcome of the previous main election. But other statistical calculations – e.g. of percentage changes in vote share and majorities, or swing calculations – do use the previous main election as the baseline.
In the case of a Westminster general election or by-election, or a devolved assembly election or by-election, a first-past-the-post constituency will be regarded as being held by the party of the sitting Member rather than whichever party won it at the previous general election or main election.
In other words, if a constituency has changed hands since the previous general election or main election because of a by-election, defection or switch to independent, PA will reflect that position in its baseline data.
For example, if a seat was won by Labour at the previous general election but then lost to the Conservatives in a subsequent by-election and the Tories still held it at the time of the new general election, PA would record the seat as being held by Conservatives in its baseline data.
If Labour then regained the seat at the new general election, PA would show this as a Labour gain from Conservative.
However, others using the previous general election as their baseline for representation, disregarding any changes since then, would show such a result as a Labour hold.
By way of illustration, here is the 2017 UK general election result for Richmond Park, as reported by PA:
A swing calculation measures the shift in voter support from one party to another in an individual result or combined across all results in so far. Overall swing calculations can provide tools for establishing which seats should fall to one party or another on an even nationwide swing and can help with forecasting an election outcome.
In an individual result, a swing calculation can be done between any two parties that fielded candidates in that seat at the previous related election. The increase in the vote share of one is added to the decrease for the other and divided by two. If both parties have seen their vote shares increase or decrease, the lower figure is subtracted from the higher and divided by two.
- In the 2017 UK general election result for Richmond Park (cf Baseline Data section above), where the Conservatives gained the seat from the Lib Dems, the Tory vote share was down 13.07 percentage points on their vote share at the 2015 GE while the Lib Dem vote was up 25.80. The combined change (13.07 + 25.80) is 38.87% which when divided by two gives a swing of 19.44% from Conservative to Lib Dem (lower to higher).
- In the same poll, the result for Basingstoke (cf Calculation of vote share changes section above), a Conservative hold, saw both Labour and the Tories increase their vote shares – Labour by 8.11 percentage points and Conservatives by 4.18. The difference between the two figures (8.11 – 4.18) is 3.93% which when divided by two gives a swing of 1.96% from Conservative to Lab (lower to higher).
In PA’s individual results (apart from council elections where results comprise summaries by party), the swing calculation is performed between the winning party and the losing party where the seat has changed hands or between the winning party and the second party where the seat has not changed hands.
If the losing party did not field a candidate this time the calculation will be between the winning party and the second party – or between the winning party and the party that won the seat at the previous main election.