Lower bound of Wilson score confidence interval (3 rating scale)

wilson.py

"""
How Not To Sort By Average Rating:
http://www.evanmiller.org/how-not-to-sort-by-average-rating.html

Note: Lower bound of a 95% confidence interval
"""
import math
import unittest


def get_wilson_score(positive, negative):
    return ((positive + 1.9208) / (positive + negative) - 1.96 * math.sqrt(
            (positive * negative) / (positive + negative) + 0.9604) /
            (positive + negative)) / (1 + 3.8416 / (positive + negative))


def get_three_rating_scale_score(one, two, three):
    positives = two * 0.5 + three
    negatives = one + two * 0.5

    return get_wilson_score(positives, negatives)


class WilsonScoreTest(unittest.TestCase):
    def test_wilson_score_with_9_positives_and_1_negative_should_get_0_5958(
        self
    ):
        result = get_wilson_score(9, 1)
        self.assertEqual(result, 0.5958436145024278)

    def test_wilson_score_with_1_positive_and_0_negative_should_get_0_2065(
        self
    ):
        result = get_wilson_score(1, 0)
        self.assertEqual(result, 0.20654329147389294)

    def test_three_rating_scale_score_of_1_5_and_9_should_get_0_5137(
        self
    ):
        result = get_three_rating_scale_score(1, 5, 9)
        self.assertEqual(result, 0.5137271335506134)


if __name__ == '__main__':
    unittest.main()