Add tests for stochastic variable generation

Tests verify:
- Take-up rate parameters load correctly (EITC, SNAP, Medicaid, etc.)
- Seeded RNG produces deterministic results
- Take-up proportions match expected rates

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: 2 people

policyengine_us_data/tests/test_stochastic_variables.py

@@ -0,0 +1,110 @@
+"""Tests for stochastic variable generation in the data package.
+
+These tests verify that:
+1. Take-up rate parameters load correctly
+2. Seeded RNG produces deterministic results
+3. Take-up rates produce plausible proportions
+"""
+
+import pytest
+import numpy as np
+from policyengine_us_data.parameters import load_take_up_rate
+
+
+class TestTakeUpRateParameters:
+    """Test that take-up rate parameters load correctly."""
+
+    def test_eitc_rate_loads(self):
+        """EITC take-up rates should load and be plausible."""
+        rates = load_take_up_rate("eitc", 2022)
+        # EITC rates are by number of children: 0, 1, 2, 3+
+        assert isinstance(rates, dict) or isinstance(rates, float)
+        if isinstance(rates, dict):
+            for key, rate in rates.items():
+                assert 0 < rate <= 1
+
+    def test_snap_rate_loads(self):
+        """SNAP take-up rate should load and be plausible."""
+        rate = load_take_up_rate("snap", 2022)
+        assert 0 < rate <= 1
+
+    def test_medicaid_rate_loads(self):
+        """Medicaid take-up rate should load and be plausible."""
+        rate = load_take_up_rate("medicaid", 2022)
+        assert 0 < rate <= 1
+
+    def test_aca_rate_loads(self):
+        """ACA take-up rate should load and be plausible."""
+        rate = load_take_up_rate("aca", 2022)
+        assert 0 < rate <= 1
+
+    def test_head_start_rate_loads(self):
+        """Head Start take-up rate should load and be plausible."""
+        rate = load_take_up_rate("head_start", 2022)
+        assert 0 < rate <= 1
+
+    def test_early_head_start_rate_loads(self):
+        """Early Head Start take-up rate should load and be plausible."""
+        rate = load_take_up_rate("early_head_start", 2022)
+        assert 0 < rate <= 1
+
+    def test_dc_ptc_rate_loads(self):
+        """DC PTC take-up rate should load and be plausible."""
+        rate = load_take_up_rate("dc_ptc", 2022)
+        assert 0 < rate <= 1
+
+
+class TestSeededRandomness:
+    """Test that stochastic generation is deterministic."""
+
+    def test_same_seed_produces_same_results(self):
+        """Using the same seed should produce identical results."""
+        seed = 0
+        n = 1_000
+
+        generator1 = np.random.default_rng(seed=seed)
+        result1 = generator1.random(n)
+
+        generator2 = np.random.default_rng(seed=seed)
+        result2 = generator2.random(n)
+
+        np.testing.assert_array_equal(result1, result2)
+
+    def test_different_seeds_produce_different_results(self):
+        """Different seeds should produce different results."""
+        n = 1_000
+
+        generator1 = np.random.default_rng(seed=0)
+        result1 = generator1.random(n)
+
+        generator2 = np.random.default_rng(seed=1)
+        result2 = generator2.random(n)
+
+        assert not np.array_equal(result1, result2)
+
+
+class TestTakeUpProportions:
+    """Test that take-up rates produce plausible proportions."""
+
+    def test_take_up_produces_expected_proportion(self):
+        """Simulated take-up should match the rate approximately."""
+        rate = 0.7
+        n = 10_000
+        generator = np.random.default_rng(seed=42)
+
+        take_up = generator.random(n) < rate
+        actual_proportion = take_up.mean()
+
+        # Should be within 5 percentage points of the rate
+        assert abs(actual_proportion - rate) < 0.05
+
+    def test_boolean_generation(self):
+        """Take-up decisions should be boolean."""
+        rate = 0.5
+        n = 100
+        generator = np.random.default_rng(seed=42)
+
+        take_up = generator.random(n) < rate
+
+        assert take_up.dtype == bool
+        assert set(take_up).issubset({True, False})