From 9be3fc18ea3845b3dcc5ba7650f09f879618ee4e Mon Sep 17 00:00:00 2001
From: Anthony Volk <14987227+anth-volk@users.noreply.github.com>
Date: Mon, 5 Oct 2026 21:40:21 +0400
Subject: [PATCH 1/2] Fix Pension Credit guarantee projection

---
 changelog.d/2072.fixed.md                     |  1 +
 .../guarantee_credit/minimum_guarantee.yaml   | 16 ++++--
 .../lag_average_earnings.py                   |  7 +++
 .../gov/economic_assumptions/lagged_series.py | 14 ++++-
 policyengine_uk/tests/test_lagged_series.py   | 53 ++++++++++++++++++-
 5 files changed, 85 insertions(+), 6 deletions(-)
 create mode 100644 changelog.d/2072.fixed.md

diff --git a/changelog.d/2072.fixed.md b/changelog.d/2072.fixed.md
new file mode 100644
index 0000000000..b7375abb1d
--- /dev/null
+++ b/changelog.d/2072.fixed.md
@@ -0,0 +1 @@
+Corrects the Pension Credit standard minimum guarantee projection to follow a non-negative lagged earnings index.
diff --git a/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml b/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
index 0c0d5c29b9..a37a6d2818 100644
--- a/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
+++ b/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
@@ -1,4 +1,8 @@
-description: Minimum income guarantee (basic amount) for Pension Credit.
+description: >-
+  Minimum income guarantee (basic amount) for Pension Credit. Values after the
+  last enacted rate use a non-negative, one-year-lagged OBR average-earnings
+  index as a forecast proxy for the statutory earnings floor; they are not
+  enacted future rates.
 SINGLE:
   values:
     2015-04-06: 151.2
@@ -14,7 +18,7 @@ SINGLE:
     2025-04-01: 221.85
     2026-04-01: 238
   metadata:
-    uprating: gov.benefit_uprating_cpi
+    uprating: gov.economic_assumptions.indices.obr.nonnegative_lagged_average_earnings
 COUPLE:
   values:
     2015-04-06: 230.85
@@ -30,7 +34,7 @@ COUPLE:
     2025-04-01: 338.61
     2026-04-01: 363.25
   metadata:
-    uprating: gov.benefit_uprating_cpi
+    uprating: gov.economic_assumptions.indices.obr.nonnegative_lagged_average_earnings
 metadata:
   period: week
   label: Pension Credit minimum guarantee
@@ -40,3 +44,9 @@ metadata:
       href: https://www.legislation.gov.uk/uksi/2002/1792/regulation/6/2021-04-12
     - title: Benefit and pension rates 2026 to 2027
       href: https://www.gov.uk/government/publications/benefit-and-pension-rates-2026-to-2027/proposed-benefit-and-pension-rates-2026-to-2027
+    - title: Social Security Administration Act 1992 section 150A
+      href: https://www.legislation.gov.uk/ukpga/1992/5/section/150A
+    - title: Social Security Administration (Northern Ireland) Act 1992 section 132A
+      href: https://www.legislation.gov.uk/ukpga/1992/8/section/132A
+    - title: A14/2025 Housing Benefit uprating for the financial year ending March 2027
+      href: https://www.gov.uk/government/publications/housing-benefit-adjudication-circulars-2025/a142025-housing-benefit-uprating-for-the-financial-year-ending-march-2027
diff --git a/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py b/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
index 9153fb0521..7f3e025a8b 100644
--- a/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
+++ b/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
@@ -15,5 +15,12 @@ def add_lagged_earnings(
     add_lagged_parameter(
         obr, "average_earnings", "lagged_average_earnings", first_year=2022
     )
+    add_lagged_parameter(
+        obr,
+        "average_earnings",
+        "nonnegative_lagged_average_earnings",
+        first_year=2022,
+        minimum_value=0,
+    )
 
     return parameters
diff --git a/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py b/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
index cd55ef948f..d86b5db275 100644
--- a/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
+++ b/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
@@ -19,15 +19,25 @@ def add_lagged_parameter(
     source_name: str,
     lagged_name: str,
     first_year: int,
+    minimum_value: float | None = None,
 ) -> Parameter:
-    """Add a one-year-lagged copy of ``source_name`` as a child of ``node``."""
+    """Add a one-year-lagged copy of ``source_name`` as a child of ``node``.
+
+    If ``minimum_value`` is set, values below it are replaced with that floor.
+    """
     source = getattr(node, source_name)
 
+    def lagged_value(year: int) -> float:
+        value = source(year - 1)
+        if minimum_value is None:
+            return value
+        return max(minimum_value, value)
+
     lagged = Parameter(
         f"{node.name}.{lagged_name}",
         data={
             "values": {
-                f"{year}-01-01": source(year - 1)
+                f"{year}-01-01": lagged_value(year)
                 for year in lag_source_years(source, first_year)
             },
         },
diff --git a/policyengine_uk/tests/test_lagged_series.py b/policyengine_uk/tests/test_lagged_series.py
index 3e64f5b890..fc61142b8e 100644
--- a/policyengine_uk/tests/test_lagged_series.py
+++ b/policyengine_uk/tests/test_lagged_series.py
@@ -6,7 +6,12 @@
 index well below the series it lags.
 """
 
+from policyengine_core.parameters import Parameter, ParameterNode
+
 from policyengine_uk import system
+from policyengine_uk.parameters.gov.economic_assumptions.lagged_series import (
+    add_lagged_parameter,
+)
 
 
 def _obr(node: str):
@@ -29,6 +34,10 @@ def test_lagged_series_reach_the_end_of_their_source():
     for source_name, lagged_name in (
         ("consumer_price_index", "lagged_cpi"),
         ("average_earnings", "lagged_average_earnings"),
+        (
+            "average_earnings",
+            "nonnegative_lagged_average_earnings",
+        ),
     ):
         source = getattr(growth, source_name)
         lagged = getattr(growth, lagged_name)
@@ -48,6 +57,10 @@ def test_lagged_series_hold_the_previous_year_of_their_source():
     for source_name, lagged_name in (
         ("consumer_price_index", "lagged_cpi"),
         ("average_earnings", "lagged_average_earnings"),
+        (
+            "average_earnings",
+            "nonnegative_lagged_average_earnings",
+        ),
     ):
         source = getattr(growth, source_name)
         lagged = getattr(growth, lagged_name)
@@ -65,9 +78,47 @@ def test_the_lagged_earnings_index_keeps_growing_with_earnings():
     assert index("2039") > index("2035") > index("2030")
 
 
+def test_lagged_series_can_apply_a_nonnegative_floor():
+    node = ParameterNode(name="test", data={})
+    node.add_child(
+        "source",
+        Parameter(
+            name="test.source",
+            data={"values": {"2024-01-01": -0.02, "2025-01-01": 0.03}},
+        ),
+    )
+
+    lagged = add_lagged_parameter(
+        node,
+        "source",
+        "nonnegative_lagged_source",
+        first_year=2025,
+        minimum_value=0,
+    )
+
+    assert lagged("2025") == 0
+    assert lagged("2026") == 0.03
+
+
+def test_minimum_guarantee_uses_nonnegative_lagged_earnings():
+    parameters = system.parameters
+    guarantee = parameters.gov.dwp.pension_credit.guarantee_credit.minimum_guarantee
+    index = _obr("indices").nonnegative_lagged_average_earnings
+
+    for relation_type in ("SINGLE", "COUPLE"):
+        branch = getattr(guarantee, relation_type)
+        expected_2027 = branch("2026") * index("2027") / index("2026")
+
+        assert branch("2027") == expected_2027
+
+
 def test_lagged_parameter_names_match_their_position_in_the_tree():
     growth = _obr("yoy_growth")
 
-    for lagged_name in ("lagged_cpi", "lagged_average_earnings"):
+    for lagged_name in (
+        "lagged_cpi",
+        "lagged_average_earnings",
+        "nonnegative_lagged_average_earnings",
+    ):
         lagged = getattr(growth, lagged_name)
         assert lagged.name.endswith(f"yoy_growth.obr.{lagged_name}")

From 2ee46f850be779629e67bb79cb3f8377f2785d9f Mon Sep 17 00:00:00 2001
From: Anthony Volk <14987227+anth-volk@users.noreply.github.com>
Date: Mon, 5 Oct 2026 23:10:02 +0400
Subject: [PATCH 2/2] Fix issues from review: use statutory earnings input

---
 changelog.d/2072.fixed.md                     |  2 +-
 .../guarantee_credit/minimum_guarantee.yaml   | 10 +--
 .../triple_lock/create_triple_lock.py         | 61 ++++++++++++++++---
 .../lag_average_earnings.py                   |  7 ---
 .../gov/economic_assumptions/lagged_series.py | 14 +----
 policyengine_uk/tests/test_lagged_series.py   | 53 +---------------
 .../test_triple_lock_statutory_inputs.py      | 54 +++++++++++++++-
 7 files changed, 111 insertions(+), 90 deletions(-)

diff --git a/changelog.d/2072.fixed.md b/changelog.d/2072.fixed.md
index b7375abb1d..a93ededf80 100644
--- a/changelog.d/2072.fixed.md
+++ b/changelog.d/2072.fixed.md
@@ -1 +1 @@
-Corrects the Pension Credit standard minimum guarantee projection to follow a non-negative lagged earnings index.
+- Corrects the Pension Credit standard minimum guarantee projection to follow the statutory May-July earnings input with a non-negative floor.
diff --git a/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml b/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
index a37a6d2818..53d53a590a 100644
--- a/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
+++ b/policyengine_uk/parameters/gov/dwp/pension_credit/guarantee_credit/minimum_guarantee.yaml
@@ -1,8 +1,8 @@
 description: >-
   Minimum income guarantee (basic amount) for Pension Credit. Values after the
-  last enacted rate use a non-negative, one-year-lagged OBR average-earnings
-  index as a forecast proxy for the statutory earnings floor; they are not
-  enacted future rates.
+  last enacted rate use the non-negative May-July average weekly earnings input
+  from the annual statutory review as a forecast floor; they are not enacted
+  future rates.
 SINGLE:
   values:
     2015-04-06: 151.2
@@ -18,7 +18,7 @@ SINGLE:
     2025-04-01: 221.85
     2026-04-01: 238
   metadata:
-    uprating: gov.economic_assumptions.indices.obr.nonnegative_lagged_average_earnings
+    uprating: gov.economic_assumptions.indices.statutory_earnings_floor
 COUPLE:
   values:
     2015-04-06: 230.85
@@ -34,7 +34,7 @@ COUPLE:
     2025-04-01: 338.61
     2026-04-01: 363.25
   metadata:
-    uprating: gov.economic_assumptions.indices.obr.nonnegative_lagged_average_earnings
+    uprating: gov.economic_assumptions.indices.statutory_earnings_floor
 metadata:
   period: week
   label: Pension Credit minimum guarantee
diff --git a/policyengine_uk/parameters/gov/dwp/state_pension/triple_lock/create_triple_lock.py b/policyengine_uk/parameters/gov/dwp/state_pension/triple_lock/create_triple_lock.py
index b977aca217..007774cb82 100644
--- a/policyengine_uk/parameters/gov/dwp/state_pension/triple_lock/create_triple_lock.py
+++ b/policyengine_uk/parameters/gov/dwp/state_pension/triple_lock/create_triple_lock.py
@@ -1,4 +1,4 @@
-"""Build the State Pension uprating series from the statutory inputs.
+"""Build statutory earnings and State Pension uprating series.
 
 Each April the basic and new State Pension rise by the highest of:
 
@@ -29,10 +29,13 @@
 earnings path started from its level in the year before the guarantee first
 applied. Current law leaves it off.
 
-The output parameter ``gov.economic_assumptions.yoy_growth.triple_lock`` holds
-the rate taking effect in April of each year, keyed to 1 January as the other
-growth series are, and runs to one year past the last year of the
-economic-assumption series.
+The output parameters hold the rates taking effect in April of each year,
+keyed to 1 January as the other growth series are. ``triple_lock`` applies the
+State Pension rule. ``statutory_earnings_floor`` holds the non-negative
+earnings component for amounts, including the Pension Credit standard minimum
+guarantee, that section 150A protects without extending the triple lock to
+them. Both run to one year past the last year of the economic-assumption
+series.
 """
 
 from dataclasses import dataclass
@@ -155,6 +158,13 @@ def uprating_rates(years: Dict[int, UpratingYear]) -> Dict[int, float]:
     return rates
 
 
+def statutory_earnings_floor_rates(
+    years: Dict[int, UpratingYear],
+) -> Dict[int, float]:
+    """Return the non-negative earnings rate from each statutory review."""
+    return {year: max(inputs.earnings, 0.0) for year, inputs in years.items()}
+
+
 def _flag(parameter: Parameter, instant: str) -> bool:
     value = parameter(instant)
     return bool(value) if value is not None else False
@@ -200,22 +210,53 @@ def read_uprating_years(parameters: ParameterNode) -> Dict[int, UpratingYear]:
 
 
 def add_triple_lock(parameters: ParameterNode) -> ParameterNode:
-    """Add ``gov.economic_assumptions.yoy_growth.triple_lock``."""
-    rates = uprating_rates(read_uprating_years(parameters))
-    new_parameter = Parameter(
+    """Add the statutory earnings floor and State Pension triple lock."""
+    years = read_uprating_years(parameters)
+    triple_lock_rates = uprating_rates(years)
+    earnings_floor_rates = statutory_earnings_floor_rates(years)
+    triple_lock = Parameter(
         "gov.economic_assumptions.yoy_growth.triple_lock",
         data={
             "description": (
                 "State Pension uprating taking effect in April of each year."
             ),
-            "values": {f"{year}-01-01": rate for year, rate in rates.items()},
+            "values": {
+                f"{year}-01-01": rate for year, rate in triple_lock_rates.items()
+            },
             "metadata": {
                 "unit": "/1",
                 "label": "State Pension uprating rate",
             },
         },
     )
+    parameters.gov.economic_assumptions.yoy_growth.add_child("triple_lock", triple_lock)
+    statutory_earnings_floor = Parameter(
+        "gov.economic_assumptions.yoy_growth.statutory_earnings_floor",
+        data={
+            "description": (
+                "Non-negative May-July average weekly earnings growth used "
+                "as the statutory uprating floor under section 150A."
+            ),
+            "values": {
+                f"{year}-01-01": rate for year, rate in earnings_floor_rates.items()
+            },
+            "metadata": {
+                "unit": "/1",
+                "label": "Statutory earnings uprating floor",
+                "reference": [
+                    {
+                        "title": (
+                            "Social Security Administration Act 1992 section 150A"
+                        ),
+                        "href": (
+                            "https://www.legislation.gov.uk/ukpga/1992/5/section/150A"
+                        ),
+                    }
+                ],
+            },
+        },
+    )
     parameters.gov.economic_assumptions.yoy_growth.add_child(
-        "triple_lock", new_parameter
+        "statutory_earnings_floor", statutory_earnings_floor
     )
     return parameters
diff --git a/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py b/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
index 7f3e025a8b..9153fb0521 100644
--- a/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
+++ b/policyengine_uk/parameters/gov/economic_assumptions/lag_average_earnings.py
@@ -15,12 +15,5 @@ def add_lagged_earnings(
     add_lagged_parameter(
         obr, "average_earnings", "lagged_average_earnings", first_year=2022
     )
-    add_lagged_parameter(
-        obr,
-        "average_earnings",
-        "nonnegative_lagged_average_earnings",
-        first_year=2022,
-        minimum_value=0,
-    )
 
     return parameters
diff --git a/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py b/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
index d86b5db275..cd55ef948f 100644
--- a/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
+++ b/policyengine_uk/parameters/gov/economic_assumptions/lagged_series.py
@@ -19,25 +19,15 @@ def add_lagged_parameter(
     source_name: str,
     lagged_name: str,
     first_year: int,
-    minimum_value: float | None = None,
 ) -> Parameter:
-    """Add a one-year-lagged copy of ``source_name`` as a child of ``node``.
-
-    If ``minimum_value`` is set, values below it are replaced with that floor.
-    """
+    """Add a one-year-lagged copy of ``source_name`` as a child of ``node``."""
     source = getattr(node, source_name)
 
-    def lagged_value(year: int) -> float:
-        value = source(year - 1)
-        if minimum_value is None:
-            return value
-        return max(minimum_value, value)
-
     lagged = Parameter(
         f"{node.name}.{lagged_name}",
         data={
             "values": {
-                f"{year}-01-01": lagged_value(year)
+                f"{year}-01-01": source(year - 1)
                 for year in lag_source_years(source, first_year)
             },
         },
diff --git a/policyengine_uk/tests/test_lagged_series.py b/policyengine_uk/tests/test_lagged_series.py
index fc61142b8e..3e64f5b890 100644
--- a/policyengine_uk/tests/test_lagged_series.py
+++ b/policyengine_uk/tests/test_lagged_series.py
@@ -6,12 +6,7 @@
 index well below the series it lags.
 """
 
-from policyengine_core.parameters import Parameter, ParameterNode
-
 from policyengine_uk import system
-from policyengine_uk.parameters.gov.economic_assumptions.lagged_series import (
-    add_lagged_parameter,
-)
 
 
 def _obr(node: str):
@@ -34,10 +29,6 @@ def test_lagged_series_reach_the_end_of_their_source():
     for source_name, lagged_name in (
         ("consumer_price_index", "lagged_cpi"),
         ("average_earnings", "lagged_average_earnings"),
-        (
-            "average_earnings",
-            "nonnegative_lagged_average_earnings",
-        ),
     ):
         source = getattr(growth, source_name)
         lagged = getattr(growth, lagged_name)
@@ -57,10 +48,6 @@ def test_lagged_series_hold_the_previous_year_of_their_source():
     for source_name, lagged_name in (
         ("consumer_price_index", "lagged_cpi"),
         ("average_earnings", "lagged_average_earnings"),
-        (
-            "average_earnings",
-            "nonnegative_lagged_average_earnings",
-        ),
     ):
         source = getattr(growth, source_name)
         lagged = getattr(growth, lagged_name)
@@ -78,47 +65,9 @@ def test_the_lagged_earnings_index_keeps_growing_with_earnings():
     assert index("2039") > index("2035") > index("2030")
 
 
-def test_lagged_series_can_apply_a_nonnegative_floor():
-    node = ParameterNode(name="test", data={})
-    node.add_child(
-        "source",
-        Parameter(
-            name="test.source",
-            data={"values": {"2024-01-01": -0.02, "2025-01-01": 0.03}},
-        ),
-    )
-
-    lagged = add_lagged_parameter(
-        node,
-        "source",
-        "nonnegative_lagged_source",
-        first_year=2025,
-        minimum_value=0,
-    )
-
-    assert lagged("2025") == 0
-    assert lagged("2026") == 0.03
-
-
-def test_minimum_guarantee_uses_nonnegative_lagged_earnings():
-    parameters = system.parameters
-    guarantee = parameters.gov.dwp.pension_credit.guarantee_credit.minimum_guarantee
-    index = _obr("indices").nonnegative_lagged_average_earnings
-
-    for relation_type in ("SINGLE", "COUPLE"):
-        branch = getattr(guarantee, relation_type)
-        expected_2027 = branch("2026") * index("2027") / index("2026")
-
-        assert branch("2027") == expected_2027
-
-
 def test_lagged_parameter_names_match_their_position_in_the_tree():
     growth = _obr("yoy_growth")
 
-    for lagged_name in (
-        "lagged_cpi",
-        "lagged_average_earnings",
-        "nonnegative_lagged_average_earnings",
-    ):
+    for lagged_name in ("lagged_cpi", "lagged_average_earnings"):
         lagged = getattr(growth, lagged_name)
         assert lagged.name.endswith(f"yoy_growth.obr.{lagged_name}")
diff --git a/policyengine_uk/tests/test_triple_lock_statutory_inputs.py b/policyengine_uk/tests/test_triple_lock_statutory_inputs.py
index a3101905d0..8320947bc1 100644
--- a/policyengine_uk/tests/test_triple_lock_statutory_inputs.py
+++ b/policyengine_uk/tests/test_triple_lock_statutory_inputs.py
@@ -4,9 +4,10 @@
 earnings growth from the previous year. These tests check that forecasts of
 those inputs follow the OBR where it publishes them, fall back to
 calendar-year growth after, respond to macro scenarios applied before the
-data load, and can be set directly; and that the earnings-path guarantee
-models one reading of the plan announced in September 2026 as a
-parameter reform.
+data load, and can be set directly. They also check that Pension Credit uses
+the same May-July earnings input without inheriting the CPI and 2.5% elements
+of the State Pension triple lock, and that the earnings-path guarantee models
+one reading of the plan announced in September 2026 as a parameter reform.
 """
 
 import numpy as np
@@ -43,10 +44,21 @@ def uprating(parameters, year):
     return parameters.gov.economic_assumptions.yoy_growth.triple_lock(f"{year}-01-01")
 
 
+def statutory_earnings_floor(parameters, year):
+    return parameters.gov.economic_assumptions.yoy_growth.statutory_earnings_floor(
+        f"{year}-01-01"
+    )
+
+
 def new_state_pension_weekly(parameters, year):
     return parameters.gov.dwp.state_pension.new_state_pension.amount(f"{year}-06-01")
 
 
+def minimum_guarantee_weekly(parameters, relation_type, year):
+    guarantee = parameters.gov.dwp.pension_credit.guarantee_credit.minimum_guarantee
+    return getattr(guarantee, relation_type)(f"{year}-06-01")
+
+
 def statutory_inputs(parameters):
     return parameters.gov.economic_assumptions.statutory_uprating_inputs
 
@@ -63,6 +75,42 @@ def test_april_2027_uses_published_may_july_2026_earnings():
     )
 
 
+def test_pension_credit_uses_the_state_pension_statutory_earnings_input():
+    parameters = system.parameters
+
+    assert statutory_inputs(parameters).awe_total_pay_may_july(
+        "2026-07-01"
+    ) == pytest.approx(0.039)
+    assert statutory_earnings_floor(parameters, 2027) == pytest.approx(0.039)
+    assert minimum_guarantee_weekly(parameters, "SINGLE", 2027) == pytest.approx(
+        238 * 1.039, rel=1e-5
+    )
+    assert minimum_guarantee_weekly(parameters, "COUPLE", 2027) == pytest.approx(
+        363.25 * 1.039, rel=1e-5
+    )
+
+
+def test_pension_credit_earnings_floor_rounds_and_never_reduces_the_guarantee():
+    parameters = parameters_under(
+        {
+            f"{INPUTS}.awe_total_pay_may_july": {
+                "2027": 0.0124,
+                "2028": -0.004,
+            }
+        }
+    )
+
+    assert [
+        statutory_earnings_floor(parameters, year) for year in (2028, 2029)
+    ] == pytest.approx([0.012, 0.0])
+    for relation_type in ("SINGLE", "COUPLE"):
+        guarantee_2027 = minimum_guarantee_weekly(parameters, relation_type, 2027)
+        guarantee_2028 = minimum_guarantee_weekly(parameters, relation_type, 2028)
+        guarantee_2029 = minimum_guarantee_weekly(parameters, relation_type, 2029)
+        assert guarantee_2028 == pytest.approx(guarantee_2027 * 1.012, rel=1e-5)
+        assert guarantee_2029 == pytest.approx(guarantee_2028, rel=1e-5)
+
+
 def test_every_published_year_has_its_own_value():
     """A missing year would silently carry the previous year's figure."""
     inputs = statutory_inputs(system.parameters)