Make Pension Credit and pension-age HB projections safe under reforms

changelog.d/2092.fixed.md

@@ -0,0 +1 @@
+Made Pension Credit and pension-age Housing Benefit projections non-negative and safe under reforms, validated their common enacted base year from the YAML as loaded, and preserved Guarantee Credit boundary continuity. Forecast thresholds and protected Housing Benefit allowances follow the reformed guarantee and maximum as uprated, as on main.

docs/book/programs/gov/dwp/pension-credit.ipynb

@@ -15,13 +15,19 @@
     "  additional support to those who have some savings or a second pension.\n",
     "\n",
     "\n",
-    "Pension Credit parameters can be found in `policyengine_uk/parameters/gov/dwp/pension_credit` and logic in `policyengine_uk/variables/dwp/pension_credit.py`.\n",
+    "Pension Credit parameters can be found in `policyengine_uk/parameters/gov/dwp/pension_credit` and logic in `policyengine_uk/variables/gov/dwp/pension_credit/`.\n",
     "\n",
     "## Legislation\n",
     "\n",
     "The legal framework for Guarantee Credit is primarily defined in the legislation provided on the [The State Pension Credit Regulations 2002](https://www.legislation.gov.uk/uksi/2002/1792/regulation/6) legislation website. \n",
     "The legal framework for Savings Credit is primarily defined in the legislation provided on the [State Pension Credit Act 2002](https://www.legislation.gov.uk/ukpga/2002/16/section/3) legislation website. \n",
     "\n",
+    "## Uprating\n",
+    "\n",
+    "After the last enacted rates, the standard minimum guarantee follows the previous May–July earnings increase, with no decrease when earnings fall, under [Social Security Administration Act 1992 section 150A](https://www.legislation.gov.uk/ukpga/1992/5/section/150A). Maximum Savings Credit follows September CPI with a zero floor. The model derives the Savings Credit threshold from these amounts and the live phase-in rate, with a zero floor on the threshold. Each higher pension-age Housing Benefit allowance with an unreformed own base equals the projected guarantee plus projected maximum Savings Credit; the post-April-2021 lower allowances follow earnings. Published amounts, including rounding, remain unchanged in their enacted years.\n",
+    "\n",
+    "The projections start from the common last year with enacted guarantees, thresholds and higher Housing Benefit allowances. Forecast thresholds and protected Housing Benefit allowances follow the guarantee and maximum as general uprating projects their reformed paths, as on main, unless their own base-year value is also reformed. Threshold forecasts use the live phase-in rate; a zero rate keeps the preceding threshold and pays no Savings Credit. Explicit threshold and allowance reforms anchor their own projections, and higher allowances at or below the guarantee, including zero, can still be projected. See the [Pension Credit parameter documentation](https://github.com/PolicyEngine/policyengine-uk/blob/main/policyengine_uk/parameters/gov/dwp/pension_credit/README.md) for the base-year checks and reform rules. These are forecast conventions, rather than announced future rates.\n",
+    "\n",
     "## Pension Credit Rate Changes\n",
     "\n",
     "Some key components of Guarantee Credit include:\n",

docs/book/programs/gov/dwp/state-pension.md

@@ -257,6 +257,12 @@ statutory-basis forecast minus the calendar-year growth stored in
   OBR does not forecast the ONS AWE series; its measure is wages and
   salaries per employee, and Q2 is the quarter nearest May to July.
 
+The State Pension triple lock consumes both statutory inputs. The shared
+`indices.earnings_uprating` series also projects the Pension Credit standard
+minimum guarantee, with a zero floor on earnings growth. See
+[Pension Credit uprating](./pension-credit.ipynb#uprating) for its Savings Credit
+and pension-age Housing Benefit projections and reform behaviour.
+
 After the EFO horizon the gap is zero, so the inputs follow calendar-year
 growth. Smooth forecasts pay the higher of earnings, CPI and 2.5% each year,
 so the baseline has none of the extra cost the triple lock builds up when

myst.yml

@@ -38,6 +38,7 @@ project:
     - title: Programmes
       children:
       - file: docs/book/programs/contrib/ubi-center/carbon-tax.ipynb
+      - file: docs/book/programs/gov/dwp/pension-credit.ipynb
       - pattern: docs/book/programs/gov/*/*.md
 site:
   options:

policyengine_uk/parameters/gov/dwp/carers_allowance/create_earnings_limit_projection.py

@@ -23,16 +23,39 @@
 """
 
 import math
+from typing import Any, Callable
 
-from policyengine_core.parameters import ParameterNode
+from policyengine_core.parameters import Parameter, ParameterNode
+from policyengine_core.parameters.parameter_at_instant import ParameterAtInstant
 from policyengine_core.periods import instant
 
 from policyengine_uk.parameters.gov.dwp.pension_credit.savings_credit.create_savings_credit_threshold import (
-    extend_after_last_stated,
     last_projected_year,
 )
 
 
+def extend_after_last_stated(
+    parameter: Parameter,
+    last_year: int,
+    derive: Callable[[str, Any], Any],
+) -> None:
+    """Add a value for each year after the parameter's last stated value.
+
+    ``derive(sample, previous)`` gives the year's value from the other
+    parameters at its 30 April ``sample``; ``previous`` is the value in force
+    before it. Each value applies from 1 January, as uprating's do.
+    """
+    last = max(parameter.values_list, key=lambda entry: entry.instant_str)
+    previous = last.value
+    for year in range(int(last.instant_str[:4]) + 1, last_year + 1):
+        previous = derive(f"{year}-04-30", previous)
+        parameter.values_list.append(
+            ParameterAtInstant(parameter.name, f"{year}-01-01", data=previous)
+        )
+    parameter.values_list.sort(key=lambda entry: entry.instant_str, reverse=True)
+    parameter.mark_as_modified()
+
+
 def national_living_wage_limit(hours: float, national_living_wage: float) -> float:
     """``hours`` at the national living wage, rounded up to the next pound.
 

policyengine_uk/parameters/gov/dwp/housing_benefit/allowances/couple/aged.yaml

@@ -1,4 +1,6 @@
-description: Personal allowance for Housing Benefit for a couple where one or both members attained State Pension age before 1 April 2021 (before that date, every couple with a member over State Pension age except, until 6 December 2018, one where both members were under 65).
+description: >-
+  Personal allowance for Housing Benefit for a couple where one or both members attained State Pension age before 1 April 2021 (before that date, every couple with a member over State Pension age except, until 6 December 2018, one where both members were under 65).
+  Values after the last enacted rate are forecasts, not enacted future rates.
 values:
   # 2015-16 to 2019-20: the rate set by the instrument cited at each value
   # (to 2018-19, the aged-65-or-over rate; younger claimants got
@@ -36,17 +38,27 @@ values:
   2022-04-06: 294.9
   2023-04-01: 324.7
   2024-04-01: 352
-  2025-04-01: 366
+  2025-04-01:
+    value: 366
+    metadata:
+      reference:
+        - title: The Social Security Benefits Up-rating Order 2025, Schedule 6 paragraph 1(2)(b)
+          href: https://www.legislation.gov.uk/uksi/2025/295/schedule/6/made
+        - title: The Social Security Benefits Up-rating Order (Northern Ireland) 2025, Schedule 6 paragraph 1(2)(b)
+          href: https://www.legislation.gov.uk/nisr/2025/64/schedule/6/made
+        - title: Benefit and pension rates 2025 to 2026, Housing Benefit personal allowances
+          href: https://www.gov.uk/government/publications/benefit-and-pension-rates-2025-to-2026/benefit-and-pension-rates-2025-to-2026
   2026-04-01: 383.35
 
 metadata:
   period: week
   unit: currency-GBP
   label: Housing Benefit couple pension-age personal allowance
-  # Not uprated as a whole: the allowance is the Pension Credit standard
+  # Component-weighted forecast: the allowance is the Pension Credit standard
   # minimum guarantee plus the maximum Savings Credit (HB A3/2021), which
   # rise by earnings and September CPI. create_pension_age_allowances.py
-  # derives the years after the published amounts as that sum.
+  # builds an allowance-specific index from that sum after published amounts.
+  uprating: gov.dwp.housing_benefit.allowances.protected_pension_age_uprating.couple
   reference:
     - title: The Housing Benefit (Persons who have attained the qualifying age for state pension credit) Regulations 2006 (SI 2006/214), Schedule 3, paragraph 1(2)(b)
       href: https://www.legislation.gov.uk/uksi/2006/214/schedule/3