code: def _sd_of_runs(stats, mean, key='runs'):
      
  
      num_runs = len(stats[key])
      first = stats[key][0]
  
      standard_deviation = {}
      for stat_key in first:
          # Skip non numberic attributes
          if isinstance(first[stat_key], numbers.Number):
              standard_deviation[stat_key] = math.sqrt(
                  sum((run[stat_key] - mean[stat_key])**2
                      for run in stats[key]) / float(num_runs))
  
      return standard_deviation
docs: Obtain the standard deviation of stats.
  
      Args:
          stats: dict; A set of stats, structured as above.
          mean: dict; Mean for each key in stats.
          key: str; Optional key to determine where list of runs is found in stats