PolicyEngine / PolicyEngine/policyengine-uk-data
Scottish/Welsh council tax ~20% above devolved budget figures; WAS owned_land imputation unstable across releases
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Description
Two data-quality issues surfaced while auditing the geographic results of the council-tax-to-LVT paper (PolicyEngine/uk-lvt-paper). Both materially distort nation-level distributional results.
1. Scottish and Welsh council tax overstated
On enhanced_frs_2023_24.h5@1.55.10 (2026, policyengine-uk 2.88.20), modelled council_tax aggregates by nation:
| Nation | Model gross | Model net | Official comparator |
|---|---|---|---|
| England | £53.8bn | £50.5bn | ≈£45.6bn requirement 2025–26 (within a few % once uprated to 2026–27) |
| Scotland | £4.8bn | £4.0bn | ≈£3.1–3.4bn |
| Wales | £3.3bn | £3.1bn | ≈£2.4–2.6bn |
The same pattern holds on enhanced_frs_2024_25.h5 (Scotland £4.3bn gross). Scottish and Welsh liabilities run roughly 20–30% above devolved budget figures while England is close. Average modelled net bill in Scotland (£1,458) is implausibly near England's (£1,872) given Scottish band D (~£1,421 in 2024–25, ex-water) and Scotland's band distribution being skewed to A–C. Possible causes worth checking: band-rate averages for Scotland including water/sewerage charges, or the VOA/SAA band calibration weighting.
Downstream effect: any reform crediting households with their abolished council tax (e.g. an LVT swap) overstates Scottish/Welsh gains by £200–300/household/year — enough to flip marginal constituencies from loss to gain.
2. owned_land (WAS direct land holdings) is large and unstable across releases
household_land_value = property_wealth × λ_r + owned_land. The owned_land component:
| Release | UK total | Wales avg/hh | Wales share of Welsh household land | Scotland share |
|---|---|---|---|---|
| enhanced_frs_2023_24@1.55.10 | £0.67tn | £53,548 | 43.6% | 27.6% |
| enhanced_frs_2024_25 (local) | £0.46tn | £11,680 | 9.8% | 31.7% |
A 4.6× swing in average Welsh direct landholding between adjacent releases points to a handful of large WAS farm/estate records being imputed into different donors each time. At £0.46–0.67tn this is 9–12% of household land — not noise. Suggest: winsorising the WAS land-value donor variable, imputing it separately from the main wealth block, or at least a regression test on nation-level owned_land aggregates.
Related: PolicyEngine/policyengine-uk#1808 (NI land intensity 0.673). Context: PolicyEngine/uk-lvt-paper#4.
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Research direction
Reproduce the nation-level council_tax and owned_land aggregates from enhanced_frs_2023_24.h5@1.55.10 and enhanced_frs_2024_25, separating Scotland, Wales, England, and the Wales and Scotland shares. Trace the WAS direct landholding imputation and the council-tax band-rate or calibration inputs, then add regression coverage for the reported aggregates across releases; done means the discrepancies are explained and the relevant totals remain stable or are intentionally documented.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- data-engineering
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100