By-election par scores

Which seat is having a by-election?

Choose a seat to predict:

Enter polling figures for Great Britain

Cons.

Lab.

Lib. Dem.

SNP + Plaid

All others

× The polling numbers you've entered add up to more than 100. Please try again.

× The polling numbers for the Conservative party are outside the range of postwar polling (17% to 57%). Forecasts may not be realistic.

× The polling numbers for the Labour party are outside the range of postwar polling (18% to 63%). Forecasts may not be realistic.

× The polling numbers for the Liberal Democrats are outside the range of postwar polling (2% to 51%). Forecasts may not be realistic.

× The polling numbers for the SNP and Plaid Cymru are greater than the Scottish and Welsh share of the GB population (13%). Forecasts may not be realistic.

× The polling numbers for all other parties are outside the range of postwar polling. Forecasts may not be realistic.

Quantity	Con	Lab	Lib Dem	SNP/ Plaid	Others
Last time out...
Best guess
Could be as low as...
Could be as high as...
Win probability

What does this site do?

This site produces forecasts of by-election results given information about each seat and current polling. These forecasts are described as “par scores” because they give information on the kind of performance each party might expect, even if their performance will be better or worse than this expectation.

What are these numbers based on?

They are based on a Dirichlet regression model of by-election outcomes in the post-war period. The model is described in the paper by Chris Hanretty, “Forecasting multiparty by-elections using Dirichlet regression.” International Journal of Forecasting 37, no. 4 (2021): 1666-1676.

What goes into these predictions?

The result in the seat in the previous general election; whether or not there is a candidate from each party; the identity of the party in government; which party held the seat; turnout in the previous general election; whether the incumbent MP is running again; polling changes since the last election.

What doesn’t go into these predictions?

Whether or not the previous MP resigned as a result of scandal; the number of “other” candidates; any other factors which might affect the local popularity of a party that aren’t reflected in national polls.

What do you mean, “could be as low as”?

All models have uncertainty in them. When we say that the result could be as low as (say) 6%, we mean that there is a 2.5% chance that the true result will be lower than this number. Conversely, when we say that the result could be as high as (say) 48%, we mean that there is a 2.5% chance that the true result will be higher than this number. Put differently, the probability that the result will fall in between these two figures is 95% (if the model is right).

Come on, I could do better than this!

We’re sure you could. After all, you might know the circumstances under which the by-election will be called, additional information about the parties’ local strengths, even the characteristics of the candidates. These are all things that the model doesn’t know. The model does know about the polling you input, historical results, and who is in government.