Data quality, backtest
GE2024 backtest
Predicting GE2024 from the GE2019 baseline and the actual GE2024 polling, measuring whether our swing model adds value over Uniform National Swing.
Generated 2026-05-27 · 649 boundary-stable constituencies evaluated
Headline
Strong Transition Model vs Uniform National Swing
Real-polling forecast test (genuine pre-election simulation)
Uses ACTUAL May 2024 polling averages (Lab 44%, Con 24%, Reform 12%), what a forecaster running this model in May 2024 would have produced. The most honest forecaster comparison.
| Mode | Winner accuracy | Major-party MAE | Brier |
|---|---|---|---|
| Uniform National Swing (May 2024 polling) | 71.2% | 5.57pp | 0.3384 |
| Strong Transition Model (May 2024 polling) | 77.5% | 5.06pp | 0.3543 |
STM lift over UNS (real polling): 0.51pp. For comparison: Survation 2024 final MRP scored 83.8% / 5.55pp RMSE; YouGov 92% (in-campaign panel of 150k); UNS counterfactual 7.61pp RMSE.
Perfect-polling structural test (model only)
Feeds the model GE2024 actual national shares as if it had perfect foresight. Isolates the structural model quality from polling-error noise.
| Mode | Winner accuracy | Major-party MAE | Brier |
|---|---|---|---|
| Uniform National Swing | 67.3% | 5.54pp | 0.3812 |
| Strong Transition Model | 78.0% | 4.52pp | 0.3938 |
| STM + anti-attenuation | 71.2% | 4.97pp | 0.3749 |
STM lift over UNS (perfect polling): 1.02pp · Polling-error penalty: 0.54pp (extra MAE we paid for using May 2024 polling vs perfect foresight) · Anti-attenuation under-shoots even with out-of-sample calibration on GE2010+2015+2017 because pre-2017 distributions had wider between-PCON variance than 2024's fragmented 4-way race.
Per-party performance (STM)
Mean absolute error by party
| Party | MAE |
|---|---|
| DUP | 25.40pp |
| Labour | 7.68pp |
| Reform UK | 4.06pp |
| Liberal Democrats | 3.97pp |
| UUP | 3.60pp |
| Green Party | 3.52pp |
| Conservative | 3.35pp |
| Independent | 2.70pp |
| Other | 2.54pp |
| The Yorkshire Party | 1.96pp |
| Alliance | 1.33pp |
| Workers Party of Britain | 1.28pp |
Worst residuals
Where we missed
| Constituency | Country | Major-party MAE |
|---|---|---|
| North Shropshire | england | 16.48pp |
| Bradford West | england | 14.66pp |
| Bristol Central | england | 14.65pp |
| Finchley and Golders Green | england | 13.39pp |
| Birmingham Hall Green and Moseley | england | 13.29pp |
| Doncaster North | england | 13.29pp |
| Bethnal Green and Stepney | england | 12.56pp |
| Manchester Rusholme | england | 12.49pp |
| Islington North | england | 12.40pp |
| Chelsea and Fulham | england | 12.33pp |