Many observers have suggested May 2015 is a unique and unpredictable election. But what is the probability that the 2015 election really will be exceptional?
Using our Election Forecast model, we can look at whether four electoral records could be broken in May. One way of understanding how our model works is to imagine that we re-run the forthcoming election many, many times, and each time we record the result.
There is considerable variation in the results (reflecting our uncertainty in specific election outcomes), but the graphs below present the relevant probabilities calculated from thousands of simulated elections.
The higher the bar, the more likely a particular result. To make things clearer, results that beat a previous record are coloured red, and non-record-breaking results are coloured gray.
So, more red in a plot suggests that more of our simulated elections were record-breaking, and thus that there is a greater probability that a given record will be broken in the forthcoming election.
1) Will the Labour and Conservatives share of the popular vote reach a record low?
The combined vote share of the Conservatives and Labour has declined gradually over the past fifty years. The rise of Ukip, SNP and Greens could mean a continuation of this trend in 2015.
In 71 per cent of the elections we simulate, the top two parties receive a smaller fraction of the popular vote than in any election since 1945. In other words, our model implies that there is an 71% probability that the combined Labour and Conservative vote share will reach a new record low.
2) Will the Labour, Conservative, and Liberal democrat share of the popular vote reach a record low?
Our model is even more confident that the combined vote share of the top three parties will be lower than ever before. The low polling figures of the Liberal Democrats (9.9 per centof the vote in our latest “Nowcast”), in addition to the likely decline of the Labour and Conservative share of the vote, means that we are almost certain that a new record will be set here.
Our model calculates the probability that this record will fall in May to be 100%. This means that the combined vote share of these three parties in our simulated elections never exceeded the historic low of 90% (a record set in the 2010 election).
3) Will Labour’s Scottish vote share reach a record low?
A key question for the election outcome is whether Labour will lose their traditional support base in Scotland. Our model suggests that it is very likely that the Labour Party will receive its lowest ever share of the Scottish vote, which was the 35.1 per cent they won in the 1983 election.
We estimate that the probability of this happening is 80 per cent.
4) Will a constituency be decided by a record-low vote share?
Under the UK’s first-past-the-post system, the candidate in each constituency winning the most votes gets elected. A potential implication of growing vote fragmentation in the UK is that, in some seats, fewer votes will be needed to win than ever before.
The record lowest winning vote share in any constituency since 1945 was in Inverness, Nairn and Lochaber in 1992 where the Lib Dems won the seat with only 26 per cent of the vote.
Our model suggests that it is quite possible (25 per cent probability) that this record will also fall in 2015. In our simulated election, there are four constituencies that are frequently decided by a record-low vote share: three in Wales (Aberconwy, Camarthen East and Dinefwr, and Dwyfor Meirionnydd) and one in Scotland (Edinburgh West).
As things stand, it is probable that the 2015 election may break a number of electoral records. We have outlined only a few of the records that may be re-made in May, but even this short list suggests that the election is likely to represent a change from the past.
Election Forecast is one of the prediction models tracked by Florence, May2015’s election-forecasting machine. Election Forecast is run by Chris Hanretty, Benjamin Lauderdale, Nick Vivyan and Jack Blumenau. This post originally appeared on the LSE’s general election blog.