# Senate fundamentals: factor-removal tests

October 19, 2026. Source cutoff October 18. No forecast weights changed.

## Does removing variables help?

I tested removing each factor family and all smaller combinations: 32 versions without generic-ballot or financial coverage restrictions, 64 in the generic-ballot cohort, and 128 in the matched financial cohort. Every subset within a cohort uses the same training and test races.

Across 130 races in 2016–2024, the full five-factor score is the best of all 32 fixed-25%-blend versions, with error 4.348. Removing any one factor makes it worse. Choosing the subset, shrinkage and blend using earlier validation elections gives error 4.642, versus 4.613 for the current forecast. The earlier-selected procedure does not validate a new weight.

In the generic-ballot cohort (79 races, three elections), dropping scandal news is the best fixed-blend subset but helps by only 0.000383 point. In the financial cohort (13 races, one election), the best subset drops fundraising, experience and scandal news; its error is 4.216, still above the current forecast’s 3.695. Financial significance cannot be estimated from one test election.

The best fixed subset is chosen after inspecting the evaluated elections, so it is a retrospective diagnostic. The earlier-selected procedure chooses variables before each target result, using only earlier validation elections. These are development comparisons on elections already studied; no new weights are activated. Correlated remaining inputs can retain related information, so removing a family does not isolate its causal effect. Missing source indicators are removed with their associated family.

| Factor removed | Average margin error | Improvement over full score |
|---|---|---|
| State partisan lean | 4.946 | -0.597 |
| Incumbency and Senate tenure | 4.417 | -0.069 |
| Earlier personal election performance | 4.354 | -0.006 |
| Challenger/open-seat experience | 4.365 | -0.016 |
| Scandal news | 4.354 | -0.006 |


## Sources and downloads

- [Removal-test protocol](fundamentals_ablation_protocol.json)
- [All removal-test results](fundamentals_ablation_analysis.json)
- [All subset historical predictions](fundamentals_ablation_predictions.csv)
- [Subset scores by election](fundamentals_ablation_cycle_scores.csv)
- [Subset coefficients and training years](fundamentals_ablation_coefficients.json)
- [Removal-test validation](fundamentals_ablation_validation.json)