Random Poll Fluctuations Add Interest

by Prof Richard Rose, 21 June 2024

Nothing is more boring than a headline that says: No change in Labour lead. Fortunately for the media, random fluctuations inherent in sampling methods ensure changes in Labour's lead from one day to the next. In eight polls taken in the space of three days between 12 and 14 June the Labour lead over the Conservatives has fluctuated between 17 and 25 percent, a difference of 8 percent.

While fluctuations in Labour's lead are frequent, this does not mean they are significant. After the polls forecast the wrong winner in the 1970 general election, I published a paper showing the extent to which random fluctuations in polls would make a hypothetical election interesting when there was no change in the 50-50 division of the vote between the red and blue parties. I assumed that a larger sample would show support for each of the two parties fluctuating by 3.3pc and support in an economy sample fluctuating by 4.7pc.

A party's lead is the difference between its own support and that of its opponent. In Britain's multi-party system the changes are not necessarily reciprocal since they often reflect a party gaining or losing support from third parties or the undecided. Thus, a surge or fall in support for the Liberal Democrats and Reform can result in both the Conservative and Labour parties losing support or both going up. I then used a computer to simulate ten polls each conducting five weekly surveys, five with a quality sample and five with an economy sample.

The 25 quality samples showed gaps fluctuating between a 10.4pc lead for the red party and a 10.4pc lead for the blue party. Only five showed the a gap within one percent of the zero gap between the two parties. In the 25 economy samples, the simulated gap ranged between a 15pc lead for the red party and a 15pc lead for the blue party, and four-fifths showed a gap of more than one percent between the two parties actually in a dead heat.

For the 2024 election Electoral Calculus assigns probabilities to the number of seats each party wins if the current distribution of the vote was repeated on election day.

Probability of Parliamentary outcomes 21 June 2024

On the current average of polls there is at least a 50pc probability of a Labour overall majority of at least 198 seats and a lead of 310 seats over the Conservatives. If the Conservatives achieved a late recovery and/or Labour stumbled badly, there is a one in twenty chance of Labour dropping to a bare majority dependent on Sinn Fein abstentions but still remaining 100 seats ahead of the Conservatives.

On the other hand, if the Tories made more gaffes there is a one in twenty chance that Labour would gain 499 seats and a super majority of 348 seats. The Conservatives could drop to 41 seats and become the third party, trailing the Liberal Democrats on 66 seats. While the Tories would still have dozens more seats than the Reform Party, but as long as it won Clacton, Nigel Farage would become a candidate to lead the Opposition in the Commons.

Prof Richard Rose is Britain's senior election expert, having written about 18 elections since 1959.