Narrowed expectation margins noticeably

bench/gen_bench_expectations.py

 #!/usr/bin/env python
 # Copyright (c) 2014 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.
 
 """ Generate bench_expectations file from a given set of bench data files. """
 
 import argparse
 import bench_util
 import os
@@ skipping 31 matching lines @@
 
   Args:
     benches: a list of float bench values.
 
   Returns:
     a list of float [lower_bound, upper_bound].
   """
   avg = sum(benches) / len(benches)
   squared_avg = avg ** 2
   avg_sum_squared = sum([bench**2 for bench in benches])/len(benches)
-  std_dev = (abs(avg_sum_squared - squared_avg) + 0.05*abs(avg)) ** 0.5
+  std_dev = (abs(avg_sum_squared - squared_avg) + 0.0001*abs(avg**2)) ** 0.5
 
   # If the results are normally distributed, 2 standard deviations
   # captures something like ~95% of the possible range of results I think
   return [avg - 2*std_dev, avg + 2*std_dev]
 
 
 def create_expectations_dict(revision_data_points, builder):
   """Convert list of bench data points into a dictionary of expectations data.
 
   Args:
@@ skipping 68 matching lines @@
               'expected': expected,
               'lower_bound': lower_bound,
               'upper_bound': upper_bound})
 
     with open(args.output_file, 'w') as file_handle:
       file_handle.write('\n'.join(out_lines))
 
 
 if __name__ == "__main__":
     main()