feat: compute PIP daily-living and mobility amounts from flags

changelog.d/added/pip-from-flags.md

@@ -0,0 +1 @@
+Add PIP daily-living and mobility amount computation from existing eligibility flags. Synthetic households built via `Simulation.from_situation` that set `pip_dl_std`/`pip_dl_enh`/`pip_mob_std`/`pip_mob_enh` now produce non-zero PIP amounts (using the new `PipParams` weekly rates), and PIP-rate reforms now flow through to the modelled amount on those households. FRS-recorded amounts continue to pass through unchanged.

interfaces/python/policyengine_uk_compiled/__init__.py

@@ -58,6 +58,7 @@ def print_guide():
     StampDutyParams,
     CapitalGainsTaxParams,
     WealthTaxParams,
+    PipParams,
     LabourSupplyParams,
     Parameters,
 )
@@ -107,6 +108,7 @@ def print_guide():
     "StampDutyParams",
     "CapitalGainsTaxParams",
     "WealthTaxParams",
+    "PipParams",
     "LabourSupplyParams",
     "Parameters",
 ]

interfaces/python/policyengine_uk_compiled/models.py

@@ -173,6 +173,20 @@ class WealthTaxParams(BaseModel):
     rate: Optional[float] = None
 
 
+class PipParams(BaseModel):
+    """Personal Independence Payment weekly rates.
+
+    Welfare Reform Act 2012 s.79; SI 2013/377. Set any of the four weekly
+    rates to model PIP-rate reforms; recipients are identified by the
+    `pip_dl_std` / `pip_dl_enh` / `pip_mob_std` / `pip_mob_enh` flags on
+    each Person.
+    """
+    daily_living_standard_weekly: Optional[float] = None
+    daily_living_enhanced_weekly: Optional[float] = None
+    mobility_standard_weekly:     Optional[float] = None
+    mobility_enhanced_weekly:     Optional[float] = None
+
+
 class LabourSupplyParams(BaseModel):
     """OBR labour supply elasticities (Slutsky decomposition).
 
@@ -230,6 +244,7 @@ class Parameters(BaseModel):
     stamp_duty: Optional[StampDutyParams] = None
     lbtt: Optional[StampDutyParams] = None
     ltt:  Optional[StampDutyParams] = None
+    pip:  Optional["PipParams"] = None
     wealth_tax: Optional[WealthTaxParams] = None
     labour_supply: Optional[LabourSupplyParams] = None
 

parameters/2025_26.yaml

@@ -288,6 +288,14 @@ wealth_tax:
   threshold: 10000000.0
   rate: 0.01
 
+pip:
+  # Personal Independence Payment weekly rates from 7 April 2025.
+  # Welfare Reform Act 2012 s.79 / SI 2013/377. Source: gov.uk/pip/what-youll-get.
+  daily_living_standard_weekly: 73.90
+  daily_living_enhanced_weekly: 110.40
+  mobility_standard_weekly:     29.20
+  mobility_enhanced_weekly:     77.05
+
 lha:
   # Local Housing Allowance rates for 2025/26.
   # LHA was re-frozen in April 2025 at the 2024/25 reset rates.

src/parameters/mod.rs

@@ -75,6 +75,9 @@ pub struct Parameters {
     /// for their bedroom entitlement category. Authority: HB Regs 2006 reg.13D.
     #[serde(default)]
     pub lha: Option<LhaParams>,
+    /// Personal Independence Payment weekly rates. Welfare Reform Act 2012 s.79.
+    #[serde(default)]
+    pub pip: Option<PipParams>,
     /// OBR labour supply response elasticities.
     /// When enabled, the Slutsky-decomposition elasticities from OBR (2023) are applied
     /// to estimate intensive-margin labour supply responses to tax-benefit reforms.
@@ -470,6 +473,19 @@ pub struct StampDutyParams {
 
 fn default_purchase_probability() -> f64 { 0.043 }
 
+/// Personal Independence Payment weekly component rates.
+///
+/// PIP has two components — daily living and mobility — each at a standard or
+/// enhanced rate. Welfare Reform Act 2012 s.79 / Social Security (Personal
+/// Independence Payment) Regulations 2013 (SI 2013/377). Rates uprated annually.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PipParams {
+    pub daily_living_standard_weekly: f64,
+    pub daily_living_enhanced_weekly: f64,
+    pub mobility_standard_weekly: f64,
+    pub mobility_enhanced_weekly: f64,
+}
+
 /// OBR labour supply response elasticities (Slutsky decomposition).
 ///
 /// Source: OBR (2023) "Costing a cut in National Insurance contributions: the

src/variables/benefits.rs

@@ -101,7 +101,7 @@ pub fn calculate_benunit(
     // All exempt from the benefit cap.
     let passthrough_benefits: f64 = bu.person_ids.iter().map(|&pid| {
         let p = &people[pid];
-        p.pip_daily_living + p.pip_mobility
+        pip_daily_living_amount(p, params) + pip_mobility_amount(p, params)
             + p.dla_care + p.dla_mobility
             + p.attendance_allowance
             + p.esa_contributory
@@ -378,6 +378,43 @@ pub(crate) fn uc_unearned_income(bu: &BenUnit, people: &[Person]) -> f64 {
 /// New SP started April 2016. SP age is 66. So in fiscal year Y, the cutoff
 /// is: anyone aged > 66 + (Y - 2016) was already SP-age when new SP began,
 /// and is therefore on basic SP. Everyone else on SP is on new SP.
+/// PIP daily-living component amount (annual).
+///
+/// If the person has a recorded amount (`p.pip_daily_living > 0`), returns that
+/// amount unchanged — preserves FRS-recorded values which may reflect partial-
+/// year claims, transitional protection, or amounts predating a reform. If the
+/// recorded amount is 0 but the standard or enhanced flag is set, returns the
+/// computed weekly rate × 52 from `params.pip`. Returns 0 when neither holds
+/// or when no PIP parameters are loaded.
+pub fn pip_daily_living_amount(p: &Person, params: &Parameters) -> f64 {
+    if p.pip_daily_living > 0.0 {
+        return p.pip_daily_living;
+    }
+    let pip = match &params.pip { Some(p) => p, None => return 0.0 };
+    if p.pip_dl_enh {
+        pip.daily_living_enhanced_weekly * 52.0
+    } else if p.pip_dl_std {
+        pip.daily_living_standard_weekly * 52.0
+    } else {
+        0.0
+    }
+}
+
+/// PIP mobility component amount (annual). See `pip_daily_living_amount`.
+pub fn pip_mobility_amount(p: &Person, params: &Parameters) -> f64 {
+    if p.pip_mobility > 0.0 {
+        return p.pip_mobility;
+    }
+    let pip = match &params.pip { Some(p) => p, None => return 0.0 };
+    if p.pip_mob_enh {
+        pip.mobility_enhanced_weekly * 52.0
+    } else if p.pip_mob_std {
+        pip.mobility_standard_weekly * 52.0
+    } else {
+        0.0
+    }
+}
+
 /// Calculate reform-adjusted state pension for a single person.
 /// New SP recipients get the full parameter rate; basic SP recipients get
 /// their reported amount scaled by the reform ratio.
@@ -2259,4 +2296,96 @@ mod parameter_impact_tests {
         // HB for private renter at £2500/month rent in London should be capped at 1-bed LHA £1200.81/month
         assert!(hb_private <= 1200.81 * 12.0 + 1.0, "HB should not exceed LHA cap for private renter");
     }
+
+    // ── PIP amount-from-flags ─────────────────────────────────────────────────
+
+    #[test]
+    fn pip_dl_enhanced_from_flag_when_amount_zero() {
+        let params = Parameters::for_year(2025).unwrap();
+        let mut p = Person::default();
+        p.age = 35.0;
+        p.pip_dl_enh = true;
+        // 2025/26 PIP DL enhanced: £110.40/week × 52 = £5,740.80
+        let amount = pip_daily_living_amount(&p, &params);
+        assert!((amount - 5_740.80).abs() < 0.01, "got {}", amount);
+    }
+
+    #[test]
+    fn pip_dl_standard_from_flag_when_amount_zero() {
+        let params = Parameters::for_year(2025).unwrap();
+        let mut p = Person::default();
+        p.age = 35.0;
+        p.pip_dl_std = true;
+        // £73.90 × 52 = £3,842.80
+        assert!((pip_daily_living_amount(&p, &params) - 3_842.80).abs() < 0.01);
+    }
+
+    #[test]
+    fn pip_mob_enhanced_from_flag() {
+        let params = Parameters::for_year(2025).unwrap();
+        let mut p = Person::default();
+        p.age = 35.0;
+        p.pip_mob_enh = true;
+        // £77.05 × 52 = £4,006.60
+        assert!((pip_mobility_amount(&p, &params) - 4_006.60).abs() < 0.01);
+    }
+
+    #[test]
+    fn pip_recorded_amount_overrides_flag() {
+        // FRS data: amount may differ from full annual rate (partial year, etc.).
+        let params = Parameters::for_year(2025).unwrap();
+        let mut p = Person::default();
+        p.age = 35.0;
+        p.pip_dl_enh = true;
+        p.pip_daily_living = 4_000.0;  // recorded — should pass through unchanged
+        assert_eq!(pip_daily_living_amount(&p, &params), 4_000.0);
+    }
+
+    #[test]
+    fn pip_no_flag_no_recorded_returns_zero() {
+        let params = Parameters::for_year(2025).unwrap();
+        let p = Person::default();
+        assert_eq!(pip_daily_living_amount(&p, &params), 0.0);
+        assert_eq!(pip_mobility_amount(&p, &params), 0.0);
+    }
+
+    #[test]
+    fn pip_returns_zero_when_params_missing() {
+        let mut params = Parameters::for_year(2025).unwrap();
+        params.pip = None;
+        let mut p = Person::default();
+        p.pip_dl_enh = true;
+        assert_eq!(pip_daily_living_amount(&p, &params), 0.0);
+    }
+
+    #[test]
+    fn pip_flows_into_passthrough_benefits() {
+        // A synthetic household with PIP enhanced flag should see the benefit
+        // amount appear in `total_benefits`.
+        let (params, mut p, bu, hh) = base_person_uc();
+        p.pip_dl_enh = true;
+        p.pip_mob_std = true;
+        let result = calc(&params, &[p], &bu, &hh);
+        // Passthrough = DL enhanced (£5740.80) + Mob standard (£1518.40) = £7259.20
+        let expected_passthrough = 5_740.80 + 1_518.40;
+        assert!(result.passthrough_benefits >= expected_passthrough - 0.01,
+                "passthrough_benefits = {}, expected at least {}",
+                result.passthrough_benefits, expected_passthrough);
+    }
+
+    #[test]
+    fn pip_param_change_flows_through() {
+        // Reform: doubling the DL enhanced rate should double the synthetic
+        // household's PIP DL amount.
+        let (mut params, mut p, bu, hh) = base_person_uc();
+        p.pip_dl_enh = true;
+        let baseline = calc(&params, &[p.clone()], &bu, &hh).passthrough_benefits;
+        if let Some(pip) = params.pip.as_mut() {
+            pip.daily_living_enhanced_weekly *= 2.0;
+        }
+        let reformed = calc(&params, &[p], &bu, &hh).passthrough_benefits;
+        // Reform should add another £5,740.80 of PIP DL enhanced.
+        assert!((reformed - baseline - 5_740.80).abs() < 0.01,
+                "baseline={}, reformed={}, delta={}", baseline, reformed, reformed - baseline);
+    }
 }

tests/policy/pip.yaml

@@ -0,0 +1,99 @@
+# Personal Independence Payment from eligibility flags.
+#
+# 2025/26 weekly rates (gov.uk/pip/what-youll-get):
+#   Daily living standard: £73.90  → £3,842.80/yr
+#   Daily living enhanced: £110.40 → £5,740.80/yr
+#   Mobility standard:     £29.20  → £1,518.40/yr
+#   Mobility enhanced:     £77.05  → £4,006.60/yr
+#
+# These cases set `would_claim_*: false` on the benunit to suppress modelled
+# means-tested benefits (UC/HB/PC), isolating PIP in `baseline_total_benefits`.
+
+- name: PIP DL enhanced + mobility standard from flags
+  period: 2025
+  absolute_error_margin: 1
+  input:
+    people:
+      p: { age: 35, employment_income: 0, pip_dl_enh: true, pip_mob_std: true }
+    benunits:
+      b:
+        members: [p]
+        would_claim_uc: false
+        would_claim_hb: false
+        would_claim_pc: false
+        would_claim_is: false
+        would_claim_ctc: false
+        would_claim_wtc: false
+        would_claim_esa: false
+        would_claim_jsa: false
+    households:
+      h: { members: [p], region: London }
+  output:
+    # £5,740.80 + £1,518.40 = £7,259.20
+    baseline_total_benefits: 7259
+
+- name: PIP DL standard alone
+  period: 2025
+  absolute_error_margin: 1
+  input:
+    people:
+      p: { age: 40, employment_income: 0, pip_dl_std: true }
+    benunits:
+      b:
+        members: [p]
+        would_claim_uc: false
+        would_claim_hb: false
+        would_claim_pc: false
+        would_claim_is: false
+        would_claim_ctc: false
+        would_claim_wtc: false
+        would_claim_esa: false
+        would_claim_jsa: false
+    households:
+      h: { members: [p], region: London }
+  output:
+    baseline_total_benefits: 3843
+
+- name: PIP mobility enhanced alone
+  period: 2025
+  absolute_error_margin: 1
+  input:
+    people:
+      p: { age: 40, employment_income: 0, pip_mob_enh: true }
+    benunits:
+      b:
+        members: [p]
+        would_claim_uc: false
+        would_claim_hb: false
+        would_claim_pc: false
+        would_claim_is: false
+        would_claim_ctc: false
+        would_claim_wtc: false
+        would_claim_esa: false
+        would_claim_jsa: false
+    households:
+      h: { members: [p], region: London }
+  output:
+    baseline_total_benefits: 4007
+
+- name: No PIP flags — no benefits when claiming disabled
+  period: 2025
+  absolute_error_margin: 0
+  input:
+    people:
+      p: { age: 40, employment_income: 0 }
+    benunits:
+      b:
+        members: [p]
+        would_claim_uc: false
+        would_claim_hb: false
+        would_claim_pc: false
+        would_claim_is: false
+        would_claim_ctc: false
+        would_claim_wtc: false
+        would_claim_esa: false
+        would_claim_jsa: false
+    households:
+      h: { members: [p], region: London }
+  output:
+    baseline_total_benefits: 0