Uprate the Scottish Carer Supplement by September CPI with 5p rounding

changelog.d/1957.fixed.md

@@ -0,0 +1 @@
+Uprate the Scottish Carer Supplement each April by the previous September's CPI, rounded to the nearest 5p, instead of holding it at £11.70 a week from 2027-28, and round the Carer Support Payment component's projected rate to the nearest 5p in the same way.

policyengine_uk/parameters/gov/social_security_scotland/carer_support_payment/rate.yaml

@@ -7,9 +7,26 @@ metadata:
   period: week
   unit: currency-GBP
   label: Carer Support Payment rate
-  uprating: gov.benefit_uprating_cpi
+  # September CPI and nearest-5p rounding follow Scottish Ministers' current
+  # policy; s86A leaves the choice of relevant prices to Ministers.
+  # Apply each year's rise to the previous rounded rate each April.
+  uprating:
+    parameter: gov.benefit_uprating_cpi
+    at_defined_interval:
+      start: "0001-01-01"
+      end: "0002-01-01"
+      enactment: "0002-04-01"
+    rounding:
+      type: nearest
+      interval: 0.05
   reference:
     - title: Carer Support Payment - mygov.scot
       href: https://www.mygov.scot/carer-support-payment
     - title: The Social Security (Up-rating) (Miscellaneous Amendments) (Scotland) Regulations 2026 reg. 12(3)(a)
       href: https://www.legislation.gov.uk/ssi/2026/170/regulation/12/made
+    - title: Social Security (Scotland) Act 2018 s. 86B (duty to uprate assistance)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86B
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 2.1 (12 months to September CPI, rounded to the nearest five pence)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/3/
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 3 (12 months to September CPI)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/4/

policyengine_uk/parameters/gov/social_security_scotland/carer_support_payment/supplement.yaml

@@ -7,10 +7,30 @@ metadata:
   period: week
   unit: currency-GBP
   label: Scottish Carer Supplement rate
+  # September CPI and nearest-5p rounding follow Scottish Ministers' current
+  # policy; s86A leaves the choice of relevant prices to Ministers.
+  # Apply each year's rise to the previous rounded rate each April.
+  uprating:
+    parameter: gov.benefit_uprating_cpi
+    at_defined_interval:
+      start: "0001-01-01"
+      end: "0002-01-01"
+      enactment: "0002-04-01"
+    rounding:
+      type: nearest
+      interval: 0.05
   reference:
-    - title: New rates for carer and disability payments in 2026
-      href: https://www.socialsecurity.gov.scot/news-events/news/new-payment-rates-from-april-2026
     - title: The Carer's Assistance (Miscellaneous and Consequential Amendments, Revocation, Transitional and Saving Provisions) (Scotland) Regulations 2025 reg. 3
       href: https://www.legislation.gov.uk/ssi/2025/340/made
     - title: The Social Security (Up-rating) (Miscellaneous Amendments) (Scotland) Regulations 2026 reg. 12(3)(b)
       href: https://www.legislation.gov.uk/ssi/2026/170/regulation/12/made
+    - title: Social Security (Scotland) Act 2018 s. 86A (inflation-adjusted level)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86A
+    - title: Social Security (Scotland) Act 2018 s. 86B (duty to uprate assistance)
+      href: https://www.legislation.gov.uk/asp/2018/9/section/86B
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 2.1 (12 months to September CPI, rounded to the nearest five pence)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/3/
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, section 3 (12 months to September CPI)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/4/
+    - title: Social Security Assistance in Scotland - up-rating for inflation in 2026-27, table 5 (2026-27 rates)
+      href: https://www.gov.scot/publications/social-security-assistance-scotland-up-rating-inflation-2026-27/pages/7/

policyengine_uk/tests/policy/baseline/gov/social_security_scotland/scottish_carer_supplement.yaml

@@ -0,0 +1,58 @@
+# Social Security (Scotland) Act 2018 s.86B requires Scottish Ministers to
+# bring forward legislation raising the Scottish Carer Supplement and the
+# Carer Support Payment component to at least their s.86A inflation-adjusted
+# levels. Their measure is the 12 months to September CPI, rounded to the
+# nearest 5p (Social Security Assistance in Scotland: up-rating for inflation
+# in 2026-27, sections 2 and 3). From April 2027 the model raises both each
+# April by the previous September's CPI (gov.benefit_uprating_cpi) and rounds
+# to the nearest 5p: 3.5% for April 2027 (ONS, 21 October 2026) and the OBR
+# March 2026 EFO forecast of 2.1% for April 2028.
+
+- name: Scottish Carer Supplement is 11.70 a week in 2026
+  period: 2026
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # SSI 2026/170 reg. 12(3)(b): 11.70 x 52.
+    scottish_carer_supplement: 608.40
+    carer_support_payment: 86.45 * 52
+
+- name: Scottish Carer Supplement rises by September CPI in 2027
+  period: 2027
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # 11.70 x 1.035 = 12.1095, to the nearest 5p 12.10; x 52 = 629.20.
+    scottish_carer_supplement: 12.10 * 52
+    # 86.45 x 1.035 = 89.47575, to the nearest 5p 89.50.
+    carer_support_payment: 89.50 * 52
+
+- name: Scottish Carer Supplement keeps rising in 2028
+  period: 2028
+  absolute_error_margin: 0.01
+  input:
+    age: 35
+    care_hours: 40
+    country: SCOTLAND
+  output:
+    # 12.10 x 1.021 = 12.3541, to the nearest 5p 12.35; x 52 = 642.20.
+    scottish_carer_supplement: 12.35 * 52
+    # 89.50 x 1.021 = 91.3795, to the nearest 5p 91.40.
+    carer_support_payment: 91.40 * 52
+
+- name: Carer in England gets no Scottish Carer Supplement in 2027
+  period: 2027
+  absolute_error_margin: 0
+  input:
+    age: 35
+    care_hours: 40
+    country: ENGLAND
+  output:
+    scottish_carer_supplement: 0
+    carer_support_payment: 0

policyengine_uk/tests/test_scottish_carer_supplement_uprating.py

@@ -0,0 +1,136 @@
+"""Uprating of the Scottish Carer Supplement and Carer Support Payment.
+
+Social Security (Scotland) Act 2018 s.86B requires Scottish Ministers to bring
+forward legislation raising each figure of assistance given under Chapter 2 of
+Part 2, carer's assistance included, to at least its s.86A inflation-adjusted
+level. Their measure is the 12 months to September CPI, rounded to the nearest
+5p (Social Security Assistance in Scotland: up-rating for inflation in
+2026-27, sections 2 and 3). SSI 2026/170 reg 12(3) raised the Carer Support
+Payment component from 83.30 to 86.45 and the Scottish Carer Supplement from
+11.29 to 11.70: both are the 3.8% September 2025 rate, rounded to 5p. The
+supplement had no uprating metadata, so the model froze it at 11.70 a week from
+2027 while the Carer Support Payment component kept rising.
+
+Both rates now rise as Scottish Child Payment and Pension Age Winter Heating
+Payment do: each April by the previous September's CPI
+(gov.benefit_uprating_cpi), rounded to the nearest 5p. Invariants, for every
+year from 2027 to 2039, the last year of the uprating index:
+
+1. Each April's rate is the previous rounded rate times one plus the
+   September CPI rise, rounded to the nearest 5p. The reference here compounds
+   the published 2026 rates in Decimal, independently of the index.
+2. Neither rate falls, since the rise is never negative.
+3. Metamorphic: setting the previous September's CPI to a published rate g
+   moves that April's rates to the previous rates times 1 + g, rounded to 5p,
+   and leaves the year before alone.
+
+The YAML tests in tests/policy/baseline/gov/social_security_scotland check
+that a Carer Support Payment recipient in Scotland gets 52 weeks of each rate.
+"""
+
+from decimal import ROUND_HALF_UP, Decimal
+
+import pytest
+from hypothesis import HealthCheck, assume, given, settings
+from hypothesis import strategies as st
+
+from policyengine_uk import Simulation
+from policyengine_uk.model_api import Scenario
+from policyengine_uk.system import system
+
+CARER = "gov.social_security_scotland.carer_support_payment"
+CPI_SEPTEMBER = "gov.economic_assumptions.statutory_uprating_inputs.cpi_september"
+# SSI 2026/170 reg 12(3)(a) and (b).
+RATES_2026 = {
+    f"{CARER}.supplement": Decimal("11.70"),
+    f"{CARER}.rate": Decimal("86.45"),
+}
+# Hand calculations: the 2026 rate x 1.035 (September 2026 CPI, ONS release
+# of 21 October 2026), to the nearest 5p: 11.70 -> 12.1095; 86.45 -> 89.47575.
+APRIL_2027 = {f"{CARER}.supplement": 12.10, f"{CARER}.rate": 89.50}
+YEARS = range(2027, 2040)
+FIVE_PENCE = Decimal("0.05")
+
+PERSON = {
+    "people": {"person": {"age": {2026: 40}}},
+    "benunits": {"benunit": {"members": ["person"]}},
+    "households": {"household": {"members": ["person"]}},
+}
+
+
+def nearest_5p(amount: Decimal) -> Decimal:
+    return (amount / FIVE_PENCE).quantize(Decimal(1), ROUND_HALF_UP) * FIVE_PENCE
+
+
+def weekly_rate(parameters, name, year):
+    return parameters.get_child(name)(f"{year}-04-30")
+
+
+@pytest.mark.parametrize("name", RATES_2026)
+def test_2026_rates_are_the_enacted_ones(name):
+    assert weekly_rate(system.parameters, name, 2026) == float(RATES_2026[name])
+
+
+@pytest.mark.parametrize("name, expected", APRIL_2027.items())
+def test_april_2027_rates_use_september_2026_cpi(name, expected):
+    assert weekly_rate(system.parameters, name, 2027) == pytest.approx(
+        expected, abs=1e-9
+    )
+
+
+@pytest.mark.parametrize("name", RATES_2026)
+def test_each_april_compounds_the_previous_rounded_rate(name):
+    parameters = system.parameters
+    rise = parameters.gov.economic_assumptions.yoy_growth.september_cpi_uprating
+    previous = RATES_2026[name]
+    for year in YEARS:
+        growth = 1 + Decimal(str(rise(f"{year}-01-01")))
+        expected = nearest_5p(previous * growth)
+        actual = weekly_rate(parameters, name, year)
+        assert actual == pytest.approx(float(expected), abs=1e-9), year
+        assert actual >= weekly_rate(parameters, name, year - 1), year
+        previous = expected
+
+
+def parameters_under(changes):
+    simulation = Simulation(
+        situation=PERSON,
+        scenario=Scenario(parameter_changes=changes, applied_before_data_load=True),
+    )
+    return simulation.tax_benefit_system.parameters
+
+
+@settings(
+    max_examples=8,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow],
+)
+@given(
+    year=st.integers(min_value=YEARS.start, max_value=2035),
+    september_permille=st.integers(min_value=0, max_value=120),
+)
+def test_rates_follow_the_previous_september_cpi(year, september_permille):
+    september = Decimal(september_permille) / 1000
+    expected = {}
+    for name in RATES_2026:
+        before = Decimal(f"{weekly_rate(system.parameters, name, year - 1):.2f}")
+        unrounded = before * (1 + september)
+        # The index is stored to 5 decimal places, so skip rises that land
+        # within a tenth of a step of a 5p half-way point.
+        steps = unrounded / FIVE_PENCE
+        remainder = abs(steps - steps.to_integral_value())
+        assume(Decimal("0.5") - remainder > Decimal("0.1"))
+        expected[name] = (before, nearest_5p(unrounded))
+
+    parameters = parameters_under(
+        {CPI_SEPTEMBER: {f"year:{year - 1}-09-01:1": float(september)}}
+    )
+
+    for name, (before, after) in expected.items():
+        assert weekly_rate(parameters, name, year - 1) == pytest.approx(
+            float(before), abs=1e-9
+        )
+        assert weekly_rate(parameters, name, year) == pytest.approx(
+            float(after), abs=1e-9
+        )