docs: def kurtosis(data):
      """Return the kurtosis of the data's distribution
  
      """
  
      if not data:
          raise StatisticsError('kurtosis requires at least one data point')
  
      size = len(data)
      sd = stdev(data) ** 4
  
      if not sd:
          return 0.0
  
      mn = mean(data)
      return sum(map(lambda x: ((x - mn) ** 4 / sd), data)) / size - 3