Tighten population tolerance and add fidelity tests

The weighted-UK-population drift that motivated #310 has already
dropped from ~6.5% to ~1.6% on current main as a side-effect of the
data-pipeline improvements landed yesterday (stage-2 QRF #362, TFC
target refresh #363, reported-anchor takeup #359).

Tightens `test_population` tolerance from 7 % to 3 % to lock in that
gain — any future calibration change that regresses back toward the
pre-April-2026 overshoot now trips CI instead of silently drifting.
Adds a new `test_population_fidelity.py` with four regression tests
extracted from the #310 draft:

- weighted-total ONS match (3 % tolerance)
- household-count sanity range (25-33 M)
- non-inflation guard (< 72 M)
- country-populations-sum-to-UK consistency

Does not include #310's loss-function change or Scotland target
removal; those are independent proposals and should be evaluated on
their own merits once the practical overshoot is resolved.

Co-authored-by: Vahid Ahmadi <va.vahidahmadi@gmail.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: 3 people

changelog.d/tighten-population-tests.added.md

@@ -0,0 +1 @@
+Tightened `test_population` tolerance from 7% to 3% now that the stage-2 QRF (#362), TFC target refresh (#363), and reported-anchor takeup (#359) pulled the weighted UK population overshoot from ~6.5% down to ~1.6%. Added four regression tests in `test_population_fidelity.py` (weighted-total match, household-count range, non-inflation guard, country-sum consistency) extracted from the earlier #310 draft so any future calibration drift back toward the pre-April-2026 overshoot trips CI.

policyengine_uk_data/tests/test_population.py

@@ -1,7 +1,12 @@
 def test_population(baseline):
     population = baseline.calculate("people", 2025).sum() / 1e6
-    POPULATION_TARGET = 69.5  # Expected UK population in millions, per ONS 2022-based estimate here: https://www.ons.gov.uk/peoplepopulationandcommunity/populationandmigration/populationprojections/bulletins/nationalpopulationprojections/2022based
-    # Tolerance temporarily relaxed to 7% due to calibration inflation issue #217
-    assert abs(population / POPULATION_TARGET - 1) < 0.07, (
+    POPULATION_TARGET = 69.5  # ONS 2022-based projection for 2025, millions: https://www.ons.gov.uk/peoplepopulationandcommunity/populationandmigration/populationprojections/bulletins/nationalpopulationprojections/2022based
+    # Tightened from 7% to 3% after data-pipeline improvements in April 2026
+    # (stage-2 QRF imputation #362, TFC target refresh #363, reported-anchor
+    # takeup #359) pulled the weighted UK population down from ~74M (+6.5%)
+    # to ~71M (+1.6%). 3% headroom keeps the test passing with room for
+    # normal build-to-build variance while catching any regression back to
+    # the pre-April-2026 overshoot.
+    assert abs(population / POPULATION_TARGET - 1) < 0.03, (
         f"Expected UK population of {POPULATION_TARGET:.1f} million, got {population:.1f} million."
     )

policyengine_uk_data/tests/test_population_fidelity.py

@@ -0,0 +1,69 @@
+"""Population fidelity regression tests for the calibrated dataset.
+
+Guards against the April 2026 calibration drift (issue #217) where the
+weighted UK population inflated ~6.5% above the ONS target. The drift
+was pulled back to ~1.6% by the data-pipeline improvements that landed
+in #362 (stage-2 QRF), #363 (TFC target refresh), and #359 (reported-
+anchor takeup). These tests lock in that gain so future calibration
+changes can't regress past current fidelity without a test failure.
+
+Extracted from PolicyEngine/policyengine-uk-data#310 (Vahid Ahmadi).
+"""
+
+from __future__ import annotations
+
+import warnings
+
+import numpy as np
+
+POPULATION_TARGET = 69.5  # ONS 2022-based projection for 2025, millions
+TOLERANCE = 0.03  # 3% — headroom above the ~1.6% achieved on current main
+
+
+def _raw(micro_series):
+    """Extract the raw numpy array from a MicroSeries without triggering
+    the `.values` deprecation warning."""
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore", UserWarning)
+        return np.array(micro_series.values)
+
+
+def test_weighted_population_matches_ons_target(baseline):
+    """Weighted UK population is within 3 % of the ONS projection."""
+    population = baseline.calculate("people", 2025).sum() / 1e6
+    assert abs(population / POPULATION_TARGET - 1) < TOLERANCE, (
+        f"Weighted population {population:.1f}M is >{TOLERANCE:.0%} "
+        f"from ONS target {POPULATION_TARGET:.1f}M."
+    )
+
+
+def test_household_count_reasonable(baseline):
+    """Total weighted households fall inside the ONS 25-33 M range."""
+    hw = _raw(baseline.calculate("household_weight", 2025))
+    total_hh = hw.sum() / 1e6
+    assert 25 < total_hh < 33, (
+        f"Total weighted households {total_hh:.1f}M outside 25-33M range."
+    )
+
+
+def test_population_not_inflated(baseline):
+    """Population stays below the pre-April-2026 inflated level (72 M)."""
+    population = baseline.calculate("people", 2025).sum() / 1e6
+    assert population < 72, (
+        f"Population {population:.1f}M exceeds 72M — calibration has "
+        "regressed toward the pre-#217 overshoot."
+    )
+
+
+def test_country_populations_sum_to_uk(baseline):
+    """England + Scotland + Wales + NI populations sum to the UK total."""
+    people = baseline.calculate("people", 2025)
+    country = baseline.calculate("country", map_to="person")
+
+    uk_pop = people.sum()
+    country_sum = sum(people[country == c].sum() for c in country.unique())
+
+    assert abs(country_sum / uk_pop - 1) < 0.001, (
+        f"Country populations sum to {country_sum / 1e6:.1f}M "
+        f"but UK total is {uk_pop / 1e6:.1f}M."
+    )