[telemetry] Fix computation of standard deviations

telemetry/telemetry/value/list_of_scalar_values_unittest.py

 # Copyright 2013 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.
 import os
 import unittest
 
 from telemetry import story
 from telemetry import page as page_module
 from telemetry import value
 from telemetry.value import improvement_direction
@@ skipping 87 matching lines @@
     self.assertTrue(v1.IsMergableWith(v0))
 
     vM = (list_of_scalar_values.ListOfScalarValues.
           MergeLikeValuesFromSamePage([v0, v1]))
     self.assertEquals(page0, vM.page)
     self.assertEquals('x', vM.name)
     self.assertEquals('unit', vM.units)
     self.assertEquals(value.CONCATENATE, vM.same_page_merge_policy)
     self.assertEquals(True, vM.important)
     self.assertEquals([10, 9, 9, 7, 300, 302, 303, 304], vM.values)
-    # SQRT((19/12 * 3 + 35/12 * 3)/6) = 1.5
-    self.assertAlmostEqual(1.5, vM.std)
+    # Values from the same page use regular standard deviation.
+    self.assertAlmostEqual(156.88849, vM.std, places=4)
     self.assertEquals('list-based metric', vM.description)
     self.assertEquals(improvement_direction.DOWN, vM.improvement_direction)
 
   def testListSamePageMergingWithPickFirstPolicy(self):
     page0 = self.pages[0]
     v0 = list_of_scalar_values.ListOfScalarValues(
         page0, 'x', 'unit',
         [1, 2], same_page_merge_policy=value.PICK_FIRST,
         improvement_direction=improvement_direction.UP)
     v1 = list_of_scalar_values.ListOfScalarValues(
@@ skipping 10 matching lines @@
     self.assertEquals(value.PICK_FIRST, vM.same_page_merge_policy)
     self.assertEquals(True, vM.important)
     self.assertEquals([1, 2], vM.values)
     self.assertEquals(improvement_direction.UP, vM.improvement_direction)
 
   def testListDifferentPageMerging(self):
     page0 = self.pages[0]
     page1 = self.pages[1]
     v0 = list_of_scalar_values.ListOfScalarValues(
         page0, 'x', 'unit',
-        [1, 2], same_page_merge_policy=value.CONCATENATE,
+        [10, 9, 9, 7], same_page_merge_policy=value.CONCATENATE,
         improvement_direction=improvement_direction.DOWN)
     v1 = list_of_scalar_values.ListOfScalarValues(
         page1, 'x', 'unit',
-        [3, 4], same_page_merge_policy=value.CONCATENATE,
+        [300, 302, 303, 304], same_page_merge_policy=value.CONCATENATE,
         improvement_direction=improvement_direction.DOWN)
     self.assertTrue(v1.IsMergableWith(v0))
 
     vM = (list_of_scalar_values.ListOfScalarValues.
           MergeLikeValuesFromDifferentPages([v0, v1]))
     self.assertEquals(None, vM.page)
     self.assertEquals('x', vM.name)
     self.assertEquals('unit', vM.units)
     self.assertEquals(value.CONCATENATE, vM.same_page_merge_policy)
     self.assertEquals(True, vM.important)
-    self.assertEquals([1, 2, 3, 4], vM.values)
+    self.assertEquals([10, 9, 9, 7, 300, 302, 303, 304], vM.values)
+    # Values from different pages use pooled standard deviation.
+    # SQRT((19/12 * 3 + 35/12 * 3)/6) = 1.5
+    self.assertAlmostEqual(1.5, vM.std)
     self.assertEquals(improvement_direction.DOWN, vM.improvement_direction)
 
   def testListWithNoneValueMerging(self):
     page0 = self.pages[0]
     v0 = list_of_scalar_values.ListOfScalarValues(
         page0, 'x', 'unit',
         [1, 2], same_page_merge_policy=value.CONCATENATE,
         improvement_direction=improvement_direction.UP)
     v1 = list_of_scalar_values.ListOfScalarValues(
         page0, 'x', 'unit',
@@ skipping 42 matching lines @@
     v1 = list_of_scalar_values.ListOfScalarValues(
         page0, 'x', 'unit',
         [300, 302, 303, 304], same_page_merge_policy=value.CONCATENATE,
         improvement_direction=improvement_direction.DOWN)
     self.assertTrue(v1.IsMergableWith(v0))
 
     vM = (list_of_scalar_values.ListOfScalarValues.
           MergeLikeValuesFromSamePage([v0, v1]))
     d = vM.AsDict()
     self.assertEquals(d['values'], [10, 9, 9, 7, 300, 302, 303, 304])
-    # SQRT((19/12 * 3 + 35/12 * 3)/6)
-    self.assertAlmostEqual(d['std'], 1.5)
+    # Values from the same page use regular standard deviation.
+    self.assertAlmostEqual(d['std'], 156.88849, places=4)
 
 
   def testNoneValueAsDict(self):
     v = list_of_scalar_values.ListOfScalarValues(
         None, 'x', 'unit', None, same_page_merge_policy=value.PICK_FIRST,
         important=False, none_value_reason='n',
         improvement_direction=improvement_direction.UP)
     d = v.AsDictWithoutBaseClassEntries()
 
     self.assertEquals(d, {
@@ skipping 54 matching lines @@
       'values': None,
       'std': None,
       'none_value_reason': 'n',
       'improvement_direction': improvement_direction.DOWN
     }
     v = value.Value.FromDict(d, {})
 
     self.assertTrue(isinstance(v, list_of_scalar_values.ListOfScalarValues))
     self.assertEquals(v.values, None)
     self.assertEquals(v.none_value_reason, 'n')