bpo-40290: Add zscore() to statistics.NormalDist. (GH-19547)

4 files changed, 37 insertions, 0 deletions

diff --git a/Doc/library/statistics.rst b/Doc/library/statistics.rst
index 026f4aa..38a499a 100644
--- a/Doc/library/statistics.rst
+++ b/Doc/library/statistics.rst
@@ -696,6 +696,16 @@ of applications in statistics.
         Set *n* to 100 for percentiles which gives the 99 cuts points that
         separate the normal distribution into 100 equal sized groups.
 
+    .. method:: NormalDist.zscore(x)
+
+        Compute the
+        `Standard Score <https://www.statisticshowto.com/probability-and-statistics/z-score/>`_
+        describing *x* in terms of the number of standard deviations
+        above or below the mean of the normal distribution:
+        ``(x - mean) / stdev``.
+
+        .. versionadded:: 3.9
+
     Instances of :class:`NormalDist` support addition, subtraction,
     multiplication and division by a constant.  These operations
     are used for translation and scaling.  For example:
diff --git a/Lib/statistics.py b/Lib/statistics.py
index 1e95c0b..9beafb3 100644
--- a/Lib/statistics.py
+++ b/Lib/statistics.py
@@ -999,6 +999,17 @@ class NormalDist:
         x2 = (a - b) / dv
         return 1.0 - (fabs(Y.cdf(x1) - X.cdf(x1)) + fabs(Y.cdf(x2) - X.cdf(x2)))
 
+    def zscore(self, x):
+        """Compute the Standard Score.  (x - mean) / stdev
+
+        Describes *x* in terms of the number of standard deviations
+        above or below the mean of the normal distribution.
+        """
+        # https://www.statisticshowto.com/probability-and-statistics/z-score/
+        if not self._sigma:
+            raise StatisticsError('zscore() not defined when sigma is zero')
+        return (x - self._mu) / self._sigma
+
     @property
     def mean(self):
         "Arithmetic mean of the normal distribution."
diff --git a/Lib/test/test_statistics.py b/Lib/test/test_statistics.py
index a9a427b..0e46a71 100644
--- a/Lib/test/test_statistics.py
+++ b/Lib/test/test_statistics.py
@@ -2602,6 +2602,21 @@ class TestNormalDist:
         with self.assertRaises(self.module.StatisticsError):
             NormalDist(1, 0).overlap(X)             # left operand sigma is zero
 
+    def test_zscore(self):
+        NormalDist = self.module.NormalDist
+        X = NormalDist(100, 15)
+        self.assertEqual(X.zscore(142), 2.8)
+        self.assertEqual(X.zscore(58), -2.8)
+        self.assertEqual(X.zscore(100), 0.0)
+        with self.assertRaises(TypeError):
+            X.zscore()                              # too few arguments
+        with self.assertRaises(TypeError):
+            X.zscore(1, 1)                          # too may arguments
+        with self.assertRaises(TypeError):
+            X.zscore(None)                          # non-numeric type
+        with self.assertRaises(self.module.StatisticsError):
+            NormalDist(1, 0).zscore(100)            # sigma is zero
+
     def test_properties(self):
         X = self.module.NormalDist(100, 15)
         self.assertEqual(X.mean, 100)
diff --git a/Misc/NEWS.d/next/Library/2020-04-15-16-43-48.bpo-40290.eqCMGJ.rst b/Misc/NEWS.d/next/Library/2020-04-15-16-43-48.bpo-40290.eqCMGJ.rst
new file mode 100644
index 0000000..a930cee
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-04-15-16-43-48.bpo-40290.eqCMGJ.rst
@@ -0,0 +1 @@
+Added zscore() to statistics.NormalDist().