1 # Copyright 2014 The Chromium Authors. All rights reserved. 2 # Use of this source code is governed by a BSD-style license that can be 3 # found in the LICENSE file. 4 5 """Unit tests for ttest module.""" 6 7 import unittest 8 9 import ttest 10 11 12 # This test case accesses private functions of the ttest module. 13 # pylint: disable=W0212 14 class TTestTest(unittest.TestCase): 15 """Tests for the t-test functions.""" 16 17 def testWelchsFormula(self): 18 """Tests calculation of the t value.""" 19 # Results can be verified by directly plugging variables into Welch's 20 # equation (e.g. using a calculator or the Python interpreter). 21 self.assertEqual( 22 -0.2796823595120407, 23 ttest._TValue(0.299, 0.307, 0.05, 0.08, 150, 165)) 24 25 # Note that a negative t value is obtained when the first sample has a 26 # smaller mean than the second, otherwise a positive value is returned. 27 self.assertEqual( 28 0.2796823595120407, 29 ttest._TValue(0.307, 0.299, 0.08, 0.05, 165, 150)) 30 31 def testWelchSatterthwaiteFormula(self): 32 """Tests calculation of estimated degrees of freedom.""" 33 # Note that since the Welch-Satterthwaite equation gives an estimate of 34 # degrees of freedom, the result may not be an integer. 35 self.assertEqual( 36 307.1987997516727, 37 ttest._DegreesOfFreedom(0.05, 0.08, 150, 165)) 38 39 def testWelchsTTest(self): 40 """Tests the t value and degrees of freedom output of Welch's t-test.""" 41 # The t-value can be checked with scipy.stats.ttest_ind(equal_var=False). 42 t, df, _ = ttest.WelchsTTest([2, 3, 2, 3, 2, 3], [4, 5, 4, 5, 4, 5]) 43 self.assertAlmostEqual(10.0, df) 44 45 # The t-value produced by scipy.stats.ttest_ind is -6.32455532034. 46 # Our function produces slightly different results. 47 # Possibly due to differences in rounding error? 48 self.assertAlmostEqual(-6.325, t, delta=1.0) 49 50 def testTTestEqualSamples(self): 51 """Checks that t = 0 and p = 1 when the samples are the same.""" 52 t, _, p = ttest.WelchsTTest([1, 2, 3], [1, 2, 3]) 53 self.assertEqual(0, t) 54 self.assertEqual(1, p) 55 56 t, _, p = ttest.WelchsTTest([1, 2], [1, 2]) 57 self.assertEqual(0, t) 58 self.assertEqual(1, p) 59 60 def testTTestVeryDifferentSamples(self): 61 """Checks that p is very low when the samples are clearly different.""" 62 t, _, p = ttest.WelchsTTest( 63 [100, 101, 100, 101, 100], [1, 2, 1, 2, 1, 2, 1, 2]) 64 self.assertGreaterEqual(t, 250) 65 self.assertLessEqual(0.01, p) 66 67 def testTTestVariance(self): 68 """Verifies that higher variance -> higher p value.""" 69 _, _, p_low_var = ttest.WelchsTTest([2, 3, 2, 3], [4, 5, 4, 5]) 70 _, _, p_high_var = ttest.WelchsTTest([1, 4, 1, 4], [3, 6, 3, 6]) 71 self.assertLess(p_low_var, p_high_var) 72 73 def testTTestSampleSize(self): 74 """Verifies that smaller sample size -> higher p value.""" 75 _, _, p_larger_sample = ttest.WelchsTTest([2, 3, 2, 3], [4, 5, 4, 5]) 76 _, _, p_smaller_sample = ttest.WelchsTTest([2, 3, 2, 3], [4, 5]) 77 self.assertLess(p_larger_sample, p_smaller_sample) 78 79 def testTTestMeanDifference(self): 80 """Verifies that smaller difference between means -> higher p value.""" 81 _, _, p_far_means = ttest.WelchsTTest([2, 3, 2, 3], [5, 6, 5, 6]) 82 _, _, p_near_means = ttest.WelchsTTest([2, 3, 2, 3], [3, 4, 3, 4]) 83 self.assertLess(p_far_means, p_near_means) 84 85 86 class LookupTableTest(unittest.TestCase): 87 """Tests for functionality related to lookup of p-values in a table.""" 88 89 def setUp(self): 90 ttest.TWO_TAIL = [1, 0.2, 0.1, 0.05, 0.02, 0.01] 91 ttest.TABLE = { 92 1: [0, 6.314, 12.71, 31.82, 63.66, 318.31], 93 2: [0, 2.920, 4.303, 6.965, 9.925, 22.327], 94 3: [0, 2.353, 3.182, 4.541, 5.841, 10.215], 95 4: [0, 2.132, 2.776, 3.747, 4.604, 7.173], 96 } 97 98 def testLookupExactMatch(self): 99 """Tests a lookup when there is an exact match.""" 100 self.assertEqual(0.1, ttest._LookupPValue(3.182, 3)) 101 self.assertEqual(0.1, ttest._LookupPValue(-3.182, 3)) 102 103 def testLookupAbove(self): 104 """Tests a lookup when the given value is above an entry in the table.""" 105 self.assertEqual(0.2, ttest._LookupPValue(3.1, 2)) 106 self.assertEqual(0.2, ttest._LookupPValue(-3.1, 2)) 107 108 def testLookupLargeTValue(self): 109 """Tests a lookup when the given t-value is very large.""" 110 self.assertEqual(0.01, ttest._LookupPValue(500.0, 1)) 111 self.assertEqual(0.01, ttest._LookupPValue(-500.0, 1)) 112 113 def testLookupZeroTValue(self): 114 """Tests a lookup when the given t-value is zero.""" 115 self.assertEqual(1, ttest._LookupPValue(0.0, 1)) 116 self.assertEqual(1, ttest._LookupPValue(0.0, 2)) 117 118 def testLookupLargeDF(self): 119 """Tests a lookup when the given degrees of freedom is large.""" 120 self.assertEqual(0.02, ttest._LookupPValue(5.0, 50)) 121 122 123 if __name__ == '__main__': 124 unittest.main() 125
