blob: b457d09035801af2b3af0543459ffcb82c3fa70a [file] [log] [blame]
 # Copyright 2014 The Chromium Authors. All rights reserved. # Use of this source code is governed by a BSD-style license that can be # found in the LICENSE file. """Unit tests for ttest module.""" import unittest import ttest # This test case accesses private functions of the ttest module. # pylint: disable=W0212 class TTestTest(unittest.TestCase): """Tests for the t-test functions.""" def testWelchsFormula(self): """Tests calculation of the t value.""" # Results can be verified by directly plugging variables into Welch's # equation (e.g. using a calculator or the Python interpreter). self.assertEqual( -0.2796823595120407, ttest._TValue(0.299, 0.307, 0.05, 0.08, 150, 165)) # Note that a negative t value is obtained when the first sample has a # smaller mean than the second, otherwise a positive value is returned. self.assertEqual( 0.2796823595120407, ttest._TValue(0.307, 0.299, 0.08, 0.05, 165, 150)) def testWelchSatterthwaiteFormula(self): """Tests calculation of estimated degrees of freedom.""" # Note that since the Welch-Satterthwaite equation gives an estimate of # degrees of freedom, the result may not be an integer. self.assertEqual( 307.1987997516727, ttest._DegreesOfFreedom(0.05, 0.08, 150, 165)) def testWelchsTTest(self): """Tests the t value and degrees of freedom output of Welch's t-test.""" # The t-value can be checked with scipy.stats.ttest_ind(equal_var=False). t, df, _ = ttest.WelchsTTest([2, 3, 2, 3, 2, 3], [4, 5, 4, 5, 4, 5]) self.assertAlmostEqual(10.0, df) # The t-value produced by scipy.stats.ttest_ind is -6.32455532034. # Our function produces slightly different results. # Possibly due to differences in rounding error? self.assertAlmostEqual(-6.325, t, delta=1.0) def testTTestEqualSamples(self): """Checks that t = 0 and p = 1 when the samples are the same.""" t, _, p = ttest.WelchsTTest([1, 2, 3], [1, 2, 3]) self.assertEqual(0, t) self.assertEqual(1, p) t, _, p = ttest.WelchsTTest([1, 2], [1, 2]) self.assertEqual(0, t) self.assertEqual(1, p) def testTTestVeryDifferentSamples(self): """Checks that p is very low when the samples are clearly different.""" t, _, p = ttest.WelchsTTest( [100, 101, 100, 101, 100], [1, 2, 1, 2, 1, 2, 1, 2]) self.assertGreaterEqual(t, 250) self.assertLessEqual(p, 0.01) def testTTestVariance(self): """Verifies that higher variance -> higher p value.""" _, _, p_low_var = ttest.WelchsTTest([2, 3, 2, 3], [4, 5, 4, 5]) _, _, p_high_var = ttest.WelchsTTest([1, 4, 1, 4], [3, 6, 3, 6]) self.assertLess(p_low_var, p_high_var) def testTTestSampleSize(self): """Verifies that smaller sample size -> higher p value.""" _, _, p_larger_sample = ttest.WelchsTTest([2, 3, 2, 3], [4, 5, 4, 5]) _, _, p_smaller_sample = ttest.WelchsTTest([2, 3, 2, 3], [4, 5]) self.assertLess(p_larger_sample, p_smaller_sample) def testTTestMeanDifference(self): """Verifies that smaller difference between means -> higher p value.""" _, _, p_far_means = ttest.WelchsTTest([2, 3, 2, 3], [5, 6, 5, 6]) _, _, p_near_means = ttest.WelchsTTest([2, 3, 2, 3], [3, 4, 3, 4]) self.assertLess(p_far_means, p_near_means) class LookupTableTest(unittest.TestCase): """Tests for functionality related to lookup of p-values in a table.""" def setUp(self): self.original_TWO_TAIL = ttest.TWO_TAIL self.original_TABLE = ttest.TABLE ttest.TWO_TAIL = [1, 0.2, 0.1, 0.05, 0.02, 0.01] ttest.TABLE = { 1: [0, 6.314, 12.71, 31.82, 63.66, 318.31], 2: [0, 2.920, 4.303, 6.965, 9.925, 22.327], 3: [0, 2.353, 3.182, 4.541, 5.841, 10.215], 4: [0, 2.132, 2.776, 3.747, 4.604, 7.173], } def tearDown(self): ttest.TWO_TAIL = self.original_TWO_TAIL ttest.TABLE = self.original_TABLE def testLookupExactMatch(self): """Tests a lookup when there is an exact match.""" self.assertEqual(0.1, ttest._LookupPValue(3.182, 3)) self.assertEqual(0.1, ttest._LookupPValue(-3.182, 3)) def testLookupAbove(self): """Tests a lookup when the given value is above an entry in the table.""" self.assertEqual(0.2, ttest._LookupPValue(3.1, 2)) self.assertEqual(0.2, ttest._LookupPValue(-3.1, 2)) def testLookupLargeTValue(self): """Tests a lookup when the given t-value is very large.""" self.assertEqual(0.01, ttest._LookupPValue(500.0, 1)) self.assertEqual(0.01, ttest._LookupPValue(-500.0, 1)) def testLookupZeroTValue(self): """Tests a lookup when the given t-value is zero.""" self.assertEqual(1, ttest._LookupPValue(0.0, 1)) self.assertEqual(1, ttest._LookupPValue(0.0, 2)) def testLookupLargeDF(self): """Tests a lookup when the given degrees of freedom is large.""" self.assertEqual(0.02, ttest._LookupPValue(5.0, 50)) if __name__ == '__main__': unittest.main()