Test that AFCS payments never confer severe disability; document AFCS vs AFIP (stacked on #1946)

changelog.d/afcs-severe-disability.changed.md

@@ -0,0 +1 @@
+Document that `afcs_reported` is Family Resources Survey benefit code 8 (Armed Forces Compensation Scheme payments, including guaranteed income payments, and war disablement pensions) and that armed forces independence payment is the separate `armed_forces_independence_payment`, and test that no AFCS payment other than armed forces independence payment makes anyone severely disabled or changes the Universal Credit higher disabled child addition, the CTC severely disabled child element, the WTC severe disability element or the severe disability premium.

policyengine_uk/tests/policy/baseline/finance/benefit/family/is_severely_disabled_for_benefits.yaml

@@ -68,3 +68,121 @@
   output:
     # WTC Regs 2002 reg 17(2) and Sch 2 para 6: 1,705 a year in 2024-25.
     WTC_severely_disabled_element: 1_705
+
+# Armed Forces Compensation Scheme payments other than armed forces
+# independence payment (lump sums, supplementary awards, guaranteed income
+# payments; AFCS Order 2011 art 15(1)) and war disablement pensions are in
+# afcs. They meet no paragraph of CTC reg 8 or WTC reg 17, alone or with a
+# lower award of another benefit.
+
+- name: DLA care component at the middle rate with an AFCS payment is not severe
+  period: 2025
+  input:
+    age: 10
+    dla_sc_category: MIDDLE
+    afcs_reported: 3_000
+  output:
+    is_severely_disabled_for_benefits: false
+
+- name: PIP daily living component at the standard rate with an AFCS payment is not severe
+  period: 2025
+  input:
+    age: 17
+    pip_dl_category: STANDARD
+    afcs_reported: 3_000
+  output:
+    is_severely_disabled_for_benefits: false
+
+- name: A child with an AFCS payment adds no CTC severely disabled child element
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_parent: true
+      child:
+        age: 8
+        afcs_reported: 2_000
+    benunits:
+      benunit:
+        members: [parent, child]
+        is_CTC_eligible: true
+  output:
+    CTC_severely_disabled_child_element: 0
+
+- name: An AFCS payment adds no WTC severe disability element
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      adult:
+        age: 45
+        afcs_reported: 12_000
+    benunits:
+      benunit:
+        members: [adult]
+        is_WTC_eligible: true
+  output:
+    WTC_severely_disabled_element: 0
+
+- name: WTC severe disability element for a claimant on armed forces independence payment
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      adult:
+        age: 45
+        # AFCS Order 2011 art 24A(3): 194.60 a week.
+        armed_forces_independence_payment: 10_119.20
+    benunits:
+      benunit:
+        members: [adult]
+        is_WTC_eligible: true
+  output:
+    # WTC Regs 2002 reg 17(4) and Sch 2 para 6: 1,705 a year in 2024-25.
+    WTC_severely_disabled_element: 1_705
+
+# UC Regs 2013 reg 24(2)(b) gives the higher rate of the disabled child
+# addition for the DLA care component at the highest rate, the PIP daily living
+# component at the enhanced rate (or a Scottish equivalent) or blindness; no
+# armed forces payment is listed. Higher rate 2025-26: 495.87 a month (reg 36
+# table); 495.87 * 12 = 5,950.44.
+
+- name: A qualifying young person with an AFCS payment gets no UC higher disabled child addition
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_parent: true
+      young_person:
+        age: 17
+        is_in_non_advanced_education: true
+        afcs_reported: 5_000
+    benunits:
+      benunit:
+        members: [parent, young_person]
+  output:
+    is_severely_disabled_for_benefits: [false, false]
+    uc_individual_severely_disabled_child_element: [0, 0]
+
+- name: A qualifying young person on the enhanced PIP daily living rate gets the UC higher addition
+  period: 2025
+  absolute_error_margin: 0.01
+  input:
+    people:
+      parent:
+        age: 40
+        is_parent: true
+      young_person:
+        age: 17
+        is_in_non_advanced_education: true
+        pip_dl_category: ENHANCED
+    benunits:
+      benunit:
+        members: [parent, young_person]
+  output:
+    is_severely_disabled_for_benefits: [false, true]
+    uc_individual_severely_disabled_child_element: [0, 5_950.44]

policyengine_uk/tests/test_severe_disability_afcs_properties.py

@@ -0,0 +1,178 @@
+"""Property-based tests for the severe disability test and Armed Forces
+Compensation Scheme payments.
+
+Law:
+- Child Tax Credit Regulations 2002 (SI 2002/2007) reg 8(3)-(5): a person is
+  severely disabled if the DLA care component is payable at the highest rate,
+  the PIP daily living component at the enhanced rate, or "an armed forces
+  independence payment is payable in respect of him".
+- Working Tax Credit (Entitlement and Maximum Rate) Regulations 2002 reg
+  17(2)-(4): the same, plus Attendance Allowance at the higher rate.
+- Armed forces independence payment is one of the injury benefits in the Armed
+  Forces and Reserve Forces (Compensation Scheme) Order 2011 art 15(1), next to
+  lump sums, supplementary awards and guaranteed income payments. Only it is
+  named; the afcs variables hold the other payments.
+
+Invariants, for any generated population of families:
+
+1. Differential: is_severely_disabled_for_benefits equals the tax credit
+   condition (CTC reg 8(3)-(5) with WTC reg 17(2)'s higher-rate Attendance
+   Allowance) recomputed here from the DLA care, PIP daily living and
+   Attendance Allowance categories and armed forces independence payment.
+2. Metamorphic: changing every person's AFCS payment (to zero or another
+   amount) changes neither the severe disability test nor the amounts that read
+   it: the UC higher rate disabled child addition, the CTC severely disabled
+   child element, the WTC severe disability element and the severe disability
+   premium.
+3. Monotone: adding armed forces independence payment to a person never turns
+   the severe disability test off, and turns it on.
+"""
+
+import numpy as np
+from hypothesis import HealthCheck, given, settings
+from hypothesis import strategies as st
+
+from policyengine_uk import Simulation
+
+UC_YEAR = 2025
+TAX_CREDIT_YEAR = 2024
+YEARS = (TAX_CREDIT_YEAR, UC_YEAR)
+PROPERTY_SETTINGS = settings(
+    max_examples=10,
+    deadline=None,
+    derandomize=True,
+    suppress_health_check=[HealthCheck.too_slow, HealthCheck.data_too_large],
+)
+DLA_CARE = ["NONE", "LOWER", "MIDDLE", "HIGHER"]
+ATTENDANCE_ALLOWANCE = ["NONE", "LOWER", "HIGHER"]
+PIP_DAILY_LIVING = ["NONE", "STANDARD", "ENHANCED"]
+amounts = st.one_of(st.just(0.0), st.floats(1, 40_000))
+
+
+@st.composite
+def people(draw, ages):
+    return dict(
+        age=draw(ages),
+        dla_sc_category=draw(st.sampled_from(DLA_CARE)),
+        pip_dl_category=draw(st.sampled_from(PIP_DAILY_LIVING)),
+        aa_category=draw(st.sampled_from(ATTENDANCE_ALLOWANCE)),
+        afcs_reported=draw(amounts),
+        armed_forces_independence_payment=draw(
+            st.one_of(st.just(0.0), st.just(0.0), st.floats(1, 15_000))
+        ),
+    )
+
+
+@st.composite
+def families(draw):
+    return dict(
+        adults=draw(st.lists(people(st.integers(25, 60)), min_size=1, max_size=2)),
+        # 16 to 19 year olds are in non-advanced education, so qualifying young
+        # persons; younger ones are children.
+        dependants=draw(st.lists(people(st.integers(0, 19)), max_size=2)),
+    )
+
+
+populations = st.lists(families(), min_size=10, max_size=20)
+
+
+def situation(units, afcs=None, afip=None):
+    """Build a situation; afcs/afip map a person name to a replacement."""
+    people_, benunits, households = {}, {}, {}
+    for i, unit in enumerate(units):
+        names = []
+        for role, members in (("a", unit["adults"]), ("d", unit["dependants"])):
+            for j, person in enumerate(members):
+                name = f"{role}{i}_{j}"
+                values = dict(person)
+                if afcs is not None:
+                    values["afcs_reported"] = afcs(name, values["afcs_reported"])
+                if afip is not None:
+                    values["armed_forces_independence_payment"] = afip(
+                        name, values["armed_forces_independence_payment"]
+                    )
+                if role == "a":
+                    values["is_parent"] = bool(unit["dependants"])
+                elif values["age"] >= 16:
+                    values["is_in_non_advanced_education"] = True
+                people_[name] = {k: {y: v for y in YEARS} for k, v in values.items()}
+                names.append(name)
+        benunits[f"b{i}"] = {
+            "members": names,
+            "is_CTC_eligible": {TAX_CREDIT_YEAR: True},
+            "is_WTC_eligible": {TAX_CREDIT_YEAR: True},
+        }
+        households[f"h{i}"] = {"members": names}
+    return {"people": people_, "benunits": benunits, "households": households}
+
+
+def reference_severely_disabled(units):
+    """CTC Regs 2002 reg 8(3)-(5) and WTC Regs 2002 reg 17(2)-(4), from the
+    inputs alone."""
+    flags = []
+    for unit in units:
+        for person in unit["adults"] + unit["dependants"]:
+            flags.append(
+                person["dla_sc_category"] == "HIGHER"
+                or person["pip_dl_category"] == "ENHANCED"
+                or person["aa_category"] == "HIGHER"
+                or person["armed_forces_independence_payment"] > 0
+            )
+    return np.array(flags)
+
+
+def outputs(sim):
+    return dict(
+        flag=sim.calculate("is_severely_disabled_for_benefits", UC_YEAR),
+        uc_addition=sim.calculate(
+            "uc_individual_severely_disabled_child_element", UC_YEAR
+        ),
+        ctc_element=sim.calculate(
+            "CTC_severely_disabled_child_element", TAX_CREDIT_YEAR
+        ),
+        wtc_element=sim.calculate("WTC_severely_disabled_element", TAX_CREDIT_YEAR),
+        premium=sim.calculate("severe_disability_premium", UC_YEAR),
+    )
+
+
+@PROPERTY_SETTINGS
+@given(populations)
+def test_severe_disability_matches_the_tax_credit_condition(units):
+    sim = Simulation(situation=situation(units))
+    flag = np.asarray(sim.calculate("is_severely_disabled_for_benefits", UC_YEAR))
+    np.testing.assert_array_equal(flag, reference_severely_disabled(units))
+
+
+@PROPERTY_SETTINGS
+@given(populations, st.data())
+def test_afcs_payments_never_change_severe_disability(units, data):
+    replacements = {}
+
+    def new_afcs(name, _):
+        if name not in replacements:
+            replacements[name] = data.draw(amounts, label=f"afcs {name}")
+        return replacements[name]
+
+    before = outputs(Simulation(situation=situation(units)))
+    no_afcs = outputs(Simulation(situation=situation(units, afcs=lambda n, v: 0.0)))
+    other_afcs = outputs(Simulation(situation=situation(units, afcs=new_afcs)))
+    for name, value in before.items():
+        np.testing.assert_allclose(no_afcs[name], value, atol=1e-6, err_msg=name)
+        np.testing.assert_allclose(other_afcs[name], value, atol=1e-6, err_msg=name)
+
+
+@PROPERTY_SETTINGS
+@given(populations, st.floats(1, 15_000))
+def test_armed_forces_independence_payment_confers_severe_disability(units, amount):
+    before = np.asarray(
+        Simulation(situation=situation(units)).calculate(
+            "is_severely_disabled_for_benefits", UC_YEAR
+        )
+    )
+    after = np.asarray(
+        Simulation(situation=situation(units, afip=lambda n, v: amount)).calculate(
+            "is_severely_disabled_for_benefits", UC_YEAR
+        )
+    )
+    assert after.all()
+    assert (after >= before).all()

policyengine_uk/variables/gov/dwp/afcs_reported.py

@@ -5,6 +5,14 @@ class afcs_reported(Variable):
     value_type = float
     entity = Person
     label = "Armed Forces Compensation Scheme (reported)"
+    documentation = (
+        "Reported Armed Forces Compensation Scheme and war disablement pension "
+        "payments. In the Family Resources Survey this is benefit code 8, asked "
+        "as the Armed Forces Compensation Scheme (formerly War Disablement "
+        "Pension), including Guaranteed Income Payments. Armed forces "
+        "independence payment is the separate armed_forces_independence_payment "
+        "variable."
+    )
     definition_period = YEAR
     unit = GBP
     uprating = "gov.economic_assumptions.indices.obr.consumer_price_index"

policyengine_uk/variables/gov/dwp/afip.py

@@ -3,7 +3,14 @@
 
 class armed_forces_independence_payment(Variable):
     label = "Armed Forces Independence Payment"
+    documentation = (
+        "Armed forces independence payment under article 24A of the Armed "
+        "Forces and Reserve Forces (Compensation Scheme) Order 2011, payable "
+        "to people with a guaranteed income payment at a relevant percentage "
+        "of 50% or more. It is not part of afcs."
+    )
     entity = Person
     definition_period = YEAR
     value_type = float
     unit = GBP
+    reference = "https://www.legislation.gov.uk/uksi/2011/517/article/24A"