Add calibration target for babies under 1 in Scotland (#259)

Add scotland_babies_under_1 calibration target (~46k) based on NRS Vital Events data.
This addresses overestimation of babies in Scotland (99k vs 46k actual births per year).

Source: NRS Vital Events Reference Tables 2024
https://www.nrscotland.gov.uk/publications/vital-events-reference-tables-2024/

Changes:
- demographics.csv: Add scotland_babies_under_1 entry (46k for all years)
- loss.py: Add matrix column for Scotland babies under 1
- test_scotland_babies.py: Add test to validate calibration accuracy (15% tolerance)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: vahid-ahmadi

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: minor
+  changes:
+    added:
+      - Add calibration target for babies under 1 in Scotland (~46k from NRS Vital Events)

policyengine_uk_data/storage/demographics.csv

@@ -75,6 +75,7 @@ scotland_age_60_69,person-k,ons_age_sex_region,,,,,691,702,714,727,741,755,767,,
 scotland_age_70_79,person-k,ons_age_sex_region,,,,,506,520,534,546,557,565,575,,,
 scotland_age_80_89,person-k,ons_age_sex_region,,,,,274,282,290,297,302,307,312,,,
 scotland_children_under_16,person-k,nrs_mid_year_population_estimates,,,,,904,900,896,892,888,884,880,,,
+scotland_babies_under_1,person-k,nrs_vital_events,,,,,46,46,46,46,46,46,46,,,
 scotland_households_3plus_children,household-k,scotland_census_2022,,,,,82,82,82,82,82,82,82,,,
 south_east_age_0_9,person-k,ons_age_sex_region,,,,,1047,1044,1042,1039,1034,1028,1024,,,
 south_east_age_10_19,person-k,ons_age_sex_region,,,,,1122,1119,1117,1114,1108,1102,1097,,,

policyengine_uk_data/tests/test_scotland_babies.py

@@ -0,0 +1,35 @@
+"""Test Scotland babies under 1 calibration target.
+
+Source: NRS Vital Events Reference Tables 2024
+https://www.nrscotland.gov.uk/publications/vital-events-reference-tables-2024/
+Scotland had 45,763 live births in 2024.
+"""
+
+import pytest
+
+
+@pytest.mark.xfail(
+    reason="Will pass after recalibration with new scotland_babies_under_1 target"
+)
+def test_scotland_babies_under_1(baseline):
+    """Test that babies under 1 in Scotland matches NRS birth statistics.
+
+    Target: ~46,000 babies under 1 (based on ~46k annual births)
+    Source: NRS Vital Events 2024 reports 45,763 births
+    """
+    region = baseline.calculate("region", map_to="person", period=2025)
+    age = baseline.calculate("age", map_to="person", period=2025).values
+    person_weight = baseline.calculate(
+        "person_weight", map_to="person", period=2025
+    ).values
+
+    scotland_babies = (region.values == "SCOTLAND") & (age < 1)
+    total_babies = (person_weight * scotland_babies).sum()
+
+    TARGET = 46_000  # NRS Vital Events 2024: 45,763 births
+    TOLERANCE = 0.15  # 15% tolerance
+
+    assert abs(total_babies / TARGET - 1) < TOLERANCE, (
+        f"Expected ~{TARGET/1000:.0f}k babies under 1 in Scotland, "
+        f"got {total_babies/1000:.0f}k ({total_babies/TARGET*100:.0f}% of target)"
+    )

policyengine_uk_data/utils/loss.py

@@ -272,6 +272,15 @@ def pe_count(*variables):
         scotland_children_under_16
     )
 
+    # Babies under 1 in Scotland
+    # Source: NRS Vital Events - births registered in Scotland
+    # https://www.nrscotland.gov.uk/publications/vital-events-reference-tables-2024/
+    # ~46,000 births per year (45,763 in 2024)
+    scotland_babies_under_1 = (region == "SCOTLAND") & (age < 1)
+    df["ons/scotland_babies_under_1"] = household_from_person(
+        scotland_babies_under_1
+    )
+
     # Households with 3+ children in Scotland
     # Source: Scotland Census 2022 - Household composition
     # https://www.scotlandscensus.gov.uk/census-results/at-a-glance/household-composition/