def test_variation_coefficient(self):
        self.assertIsNanOnAllArrayTypes(
            variation_coefficient, [1, 1, -1, -1],
        )
        self.assertAlmostEqualOnAllArrayTypes(
            variation_coefficient, [1, 2, -3, -1], -7.681145747868608
        )
        self.assertAlmostEqualOnAllArrayTypes(
            variation_coefficient, [1, 2, 4, -1], 1.2018504251546631
        )
        self.assertIsNanOnAllArrayTypes(variation_coefficient, [])