[telemetry] Fix computation of standard deviations

telemetry/telemetry/value/list_of_scalar_values.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 numbers
 import math
 
 from telemetry import value as value_module
 from telemetry.value import none_values
 from telemetry.value import summarizable
 
 
 def Variance(sample):
   """ Compute the population variance.
 
     Args:
       sample: a list of numbers.
   """
   k = len(sample) - 1  # Bessel correction
   if k <= 0:
-    return 0
+    return 0.0
   m = _Mean(sample)
   return sum((x - m)**2 for x in sample)/k
 
 
 def StandardDeviation(sample):
   """ Compute standard deviation for a list of numbers.
 
     Args:
       sample: a list of numbers.
   """
@@ skipping 15 matching lines @@
   total_degrees_of_freedom = 0
   for i in xrange(len(list_of_samples)):
     l = list_of_samples[i]
     k = len(l) - 1  # Bessel correction
     if k <= 0:
       continue
     variance = list_of_variances[i] if list_of_variances else Variance(l)
     pooled_variance += k * variance
     total_degrees_of_freedom += k
   if total_degrees_of_freedom:
-    return (pooled_variance/total_degrees_of_freedom) ** 0.5
-  return 0
+    return (pooled_variance / total_degrees_of_freedom) ** 0.5
+  else:
+    return 0.0
 
 
 def _Mean(values):
   return float(sum(values)) / len(values) if len(values) > 0 else 0.0
 
 
 class ListOfScalarValues(summarizable.SummarizableValue):
   """ ListOfScalarValues represents a list of numbers.
 
   By default, std is the standard deviation of all numbers in the list. Std can
@@ skipping 137 matching lines @@
     list_of_samples = []
     none_value_reason = None
     pooled_std = None
     for v in values:
       if v.values is None:
         merged_values = None
         none_value_reason = none_values.MERGE_FAILURE_REASON
         break
       merged_values.extend(v.values)
       list_of_samples.append(v.values)
-    if merged_values:
+    if merged_values and page is None:
+      # Pooled standard deviation is only used when merging values comming from
+      # different pages. Otherwise, fall back to the default computation in
+      # ListOfScalarValues' constructor.

[eakuefner, 2016/06/20 17:26:32]

supernit: s/s'/s's/

[perezju, 2016/06/21 09:48:07]

Grammar confuses me. I rephrased this to avoid the apostrophe.

       pooled_std = PooledStandardDeviation(
           list_of_samples, list_of_variances=[v.variance for v in values])
     return ListOfScalarValues(
         page, name, v0.units,
         merged_values,
         important=v0.important,
         description=v0.description,
         tir_label=tir_label,
         same_page_merge_policy=v0.same_page_merge_policy,
         std=pooled_std,
         none_value_reason=none_value_reason,
         improvement_direction=v0.improvement_direction,
         grouping_keys=grouping_keys)