docs: def variance(arr):
    """variance of the values, must have 2 or more entries.
  
    :param arr: list of numbers
    :type arr: number[] a number array
    :return: variance
    :rtype: float
  
    """
    avg = average(arr)
    return sum([(float(x)-avg)**2 for x in arr])/float(len(arr)-1)