Breaking Down the Arithmetic of a Uniquely Close U.K. Election

Polls show the top parties are virtually tied

A red London bus moves across Westminster Bridge away from the Big Ben clock tower and the Houses of Parliament in London.

Matthew Lloyd/Bloomberg

The rise of small parties in Britain complicates the mapping of vote shares to parliamentary seats in an electoral system that differs substantially from that of the U.S. Without a big swing before the May 7 election, a hung parliament looks unavoidable. Ahead of the vote, watch out for the influence party manifestos and televised debates might have on the polls. Whichever of the two main parties gains the most seats can probably claim a democratic mandate — albeit a weak one — to try to form a government. With the Conservatives and Labour whisker to whisker in the polls, it’s impossible to be confident about which one that’ll be.

 The Parties

Vote and Seat Shares, 2010 and Now

Screen Shot 2015-03-27 at 8.14.16 AM
  • Conservative Party: Center-right party led by Prime Minister David Cameron that took the most seats in 2010 and formed a coalition government with the Liberal Democrats.
  • Labour Party: Center-left opposition, led by Ed Miliband.
  • Liberal Democrats: Centrist junior coalition partner in the most recent government, led by Deputy Prime Minister Nick Clegg.
  • Scottish National Party: Largest political party in Scotland that led a close campaign for independence from the U.K. It lies on the center-left of the political spectrum and is led by Nicola Sturgeon.
  • U.K. Independence Party (UKIP): Right-wing, campaigning on withdrawal from the European Union, led by Nigel Farage.
  • Green Party: Left-wing, campaigning on environmental issues, led by Natalie Bennett.
  • Plaid Cymru: Largest party in Wales, center-left/left, led by Leanne Wood.
  • Democratic Unionist Party: Largest party in Northern Ireland led by Peter Robinson, right-wing.
  • Sinn Fein: Second-largest party in Northern Ireland, left-wing, led by Gerry Adams.

How the British Vote

Like the U.S., the U.K. has an upper house, a lower house and a leader of the executive. Unlike the U.S., the U.K.’s executive is drawn from the legislature. Britons vote for their local candidates who represent a constituency in the lower chamber of parliament — the House of Commons. The leader of the dominant party assumes the role of prime minister and the upper chamber remains unelected.

As in the U.S., the British vote for their representatives on a first-past-the-post basis — a small majority within a constituency is just as good as a large one. This system doesn’t benefit Conservatives or Labour over one another. However, compared with other systems like proportional representation, it tends to favor the major parties at the expense of smaller ones, whose support may be scattered across the country. It also encourages tactical voting — putting aside your preferred party to keep another out of government. That’s one reason why UKIP and the Green Party’s vote share will probably translate into fewer seats per vote cast than the main parties at the general election.

The electoral boundaries are also important and have traditionally supported Labour’s seat count. The boundaries are only redrawn infrequently and are based on outdated figures for local population sizes — the latest are based on numbers for the year 2000. Demographic trends mean Conservative majority constituencies now tend to contain more people than Labour constituencies, and the influence is likely to be bigger in 2015 than in 2010, since those trends have had longer to run. Also in Labour’s favor is the seat quota for Wales, where the party enjoys strong support, delivering more seats per capita than England and Scotland.

The Polls

There are five main polls in the U.K. that survey on a monthly basis at a minimum. The survey methods vary between the telephone and Internet, and efforts are made to adjust for the demographic profile of respondents to achieve a representative draw. Some were more accurate than others in predicting the 2010 election outcome, though there is no particular reason for that to be repeated this time -- the volatility of the polls just before the election suggests their accuracy could easily have been by chance. The differences between the polls are summarized in the table below.

Screen Shot 2015-03-27 at 8.42.23 AM

Polls of Polls

In America, state-by-state polling is extensive and the sample sizes are large. It is that volume of data that facilitates the sort of analysis Nate Silver of conducted that correctly predicted the 2012 election outcome in all 50 states. At 650, the number of constituencies in the U.K. is 13 times the number of American states even though the population is just one fifth of the U.S. — it would simply be too expensive to poll them all.

Instead, the British are left to puzzle over the national aggregates and look to the Ashcroft survey of marginal constituencies as a guide to the behavior of swing voters. The scope for prediction errors, and therefore election-related market reactions, is probably greater in the U.K. than it has been in the U.S. of late.

While the polls appear to be relatively stable in a band of plus or minus two percentage points, within it they are volatile, thanks partly to relatively small sample sizes. To gain a broader indicator of sentiment, polls are typically weighted together — often taking a simple average. An alternative is to apply a statistical technique known as Principal Components Analysis that attempts to extract a common underlying signal from the polls. Applying that technique to the U.K.’s five most comprehensive monthly polls and benchmarking them to the mean and standard deviation of the ComRes poll (which was not biased one way or the other in 2010) gives the results illustrated in the panel below. The technique may confer only a small advantage over the simple average with differences from it ranging between plus or minus one percentage point of the vote share.

Estimated Vote Shares

Screen Shot 2015-03-27 at 8.16.58 AM

This Bloomberg Intelligence analysis puts Labour and the Conservatives virtually tied in March. The Liberal Democrats seem roughly to be treading water against a backdrop of gently waning support.

The Conservatives look to have made up some ground over the past few months, possibly reflecting the improving economic outlook, which is important to voters. Having been vulnerable to Labour’s “cost of living crisis” narrative, the Conservatives are benefiting from the falling cost of oil lowering inflation and lifting real wage growth -- though the latter remains very weak by historical standards. Together with lower interest rates, that has also provided a fillip to the public finances. The final budget of the current parliament was published on March 18 and received generally positive press coverage. Yet that seems not to have translated into a meaningful boost to the Conservatives or the Liberal Democrats, based on the latest polls for the month of March.

The big change since the 2010 is the evaporation of Labour’s support in Scotland. The independence movement has energized the Scottish National Party, which now looks likely to claim many of Labour’s seats north of the border. The appointment of a new leader for Scottish Labour so far seems to have failed to reverse that trend.

The challenges of coalition government have not been kind to the Liberal Democrats, and they can expect to shed a large number of seats. A backtrack on a pre-2010 election pledge to prevent student tuition fees from rising left many supporters feeling betrayed, and the tie-in with the Conservatives remains toxic. The rise of UKIP has also seen Nigel Farage’s party claim a higher proportion of the expected vote share from the two biggest parties. While that won’t translate into many seats, it will unevenly sap votes from the traditional parties -- possibly tilting the balance between them in some constituencies. 

What to Watch

There are a few key events that could have a bearing on the polls in the run up to election day, and swings in the polls are not unusual just before the event, as the chart below illustrates.

Screen Shot 2015-03-27 at 8.18.21 AM

First there are the party manifestos, which may be published next week when parliament is likely to be dissolved and the short campaign is triggered. This phase of the election is when spending on the campaign ramps up. For the coalition partners, those manifestos could see more detail on how they would govern differently apart. For Labour, it could see a fleshing out of policy objectives. The most likely outcome is that each manifesto is thin on detail, particularly when it comes to public spending plans. There is a risk, however, of one party jumping the gun, and being more specific about how it would close Britain’s budget deficit, seeking a first-mover political advantage.

The 2010 televised debates had a significant effect on the polls, with a strong performance from Nick Clegg prompting a major swing toward the Liberal Democrats. This time around David Cameron has taken a much more cautious stance, rejecting proposals for a series of head-to-head debates. Instead of live debates featuring all three of the main parties, there will be live question and answer sessions and one seven-way debate. The impact may therefore be smaller than it was in 2010.

Here’s what will be taking place:

  • March 26: Live question and answer program on Channel 4 and Sky News featuring Cameron and Miliband separately, presented by Jeremy Paxman and Kay Burley.
  • April 2: Debate with seven party leaders on ITV, moderated by Julie Etchingham.
  • April 16: Debate between five opposition party leaders on the BBC, moderated by David Dimbleby.
  • April 30: BBC Question Time program with Cameron, Miliband and Clegg, presented by David Dimbleby.

Pre-election statements from the parties on possible coalitions also have the potential to affect voting behavior, particularly tactical voting. The SNP has already ruled out a coalition with the Conservatives, and the Labour Party has ruled out a formal coalition with the SNP, though not an informal agreement.


It’s important to recognize that most of the polls are conducted at a national level, so they estimate the vote share only. Even if the polls were 100 percent correct (and they vary widely), analysts would still have to map that vote share to seats. That task has become much more complicated with the movement from what was roughly a two-party system to one under which six parties might reasonably be expected to take a meaningful number of seats. That development is prompting a wide range of forecasts among pollsters making this election uniquely uncertain. With the scope for a surprise large, so too might be any market reactions associated with it after May 7.

The table below shows how a few of the main political analysts expect vote shares to translate into seats. The key thing to take from it is that under each scenario the election outcome is a hung parliament — no party would be anywhere near the 326 number of seats required to form a majority government (or 323 excluding Sinn Fein, which usually does not take its seats).

Outside Analysts’ Seat Predictions

Screen Shot 2015-03-27 at 8.20.50 AM

The Bloomberg Intelligence estimates of the vote shares can be mapped to the seat count in a relatively mechanical way by applying the average translation from political analysts between votes and seats. That would be consistent with a very narrow lead for the Conservatives, with 279 members of parliament, compared with about 270 for Labour and no more than 20 for the Liberal Democrats -- that scenario would probably see the Conservatives have first crack at forming a government. It is worth remembering that sampling error and scope for seat-mapping error mean the gap between the two main parties is nowhere near big enough to be confident of that outcome. Without a very big swing in the polls, uncertainty will remain elevated to the very last.

This post is courtesy of Bloomberg Intelligence Economics.

Before it's here, it's on the Bloomberg Terminal.