Fix randomness between runs caused by UC reform (#1359)

* Fix random seed

* Update tests

Author: nikhilwoodruff

changelog_entry.yaml

@@ -0,0 +1,4 @@
+- bump: patch
+  changes:
+    fixed:
+    - Fixed randomness between runs caused by UC reform.

policyengine_uk/scenarios/uc_reform.py

@@ -8,7 +8,9 @@ def add_universal_credit_reform(sim: Microsimulation):
         sim.tax_benefit_system.parameters.gov.dwp.universal_credit.rebalancing
     )
 
-    uc_seed = np.random.random(len(sim.calculate("benunit_id")))
+    generator = np.random.default_rng(43)
+
+    uc_seed = generator.random(len(sim.calculate("benunit_id")))
     p_uc_post_2026_status = {
         2025: 0,
         2026: 0.11,

policyengine_uk/tests/microsimulation/reforms_config.yaml

@@ -1,33 +1,33 @@
 reforms:
 - name: Raise basic rate by 1pp
-  expected_impact: 8.0
+  expected_impact: 8.1
   parameters:
     gov.hmrc.income_tax.rates.uk[0].rate: 0.21
 - name: Raise higher rate by 1pp
-  expected_impact: 5.0
+  expected_impact: 4.9
   parameters:
     gov.hmrc.income_tax.rates.uk[1].rate: 0.42
 - name: Raise personal allowance by ~800GBP/year
   expected_impact: 0.7
   parameters:
     gov.hmrc.income_tax.allowances.personal_allowance.amount: 13000
 - name: Raise child benefit by 25GBP/week per additional child
-  expected_impact: -1.5
+  expected_impact: -1.3
   parameters:
     gov.hmrc.child_benefit.amount.additional: 25
 - name: Reduce Universal Credit taper rate to 20%
-  expected_impact: -36.5
+  expected_impact: -36.0
   parameters:
     gov.dwp.universal_credit.means_test.reduction_rate: 0.2
 - name: Raise Class 1 main employee NICs rate to 10%
-  expected_impact: 12.4
+  expected_impact: 12.5
   parameters:
     gov.hmrc.national_insurance.class_1.rates.employee.main: 0.1
 - name: Raise VAT standard rate by 2pp
-  expected_impact: 21.2
+  expected_impact: 19.1
   parameters:
     gov.hmrc.vat.standard_rate: 0.22
 - name: Raise additional rate by 3pp
-  expected_impact: 5.4
+  expected_impact: 4.5
   parameters:
     gov.hmrc.income_tax.rates.uk[2].rate: 0.48