极差，方差，标准差

def Var(t, mu=None):
    """方差"""
    if mu is None:
        mu = Mean(t)

    # compute the squared deviations and return their mean.
    dev2 = [(x - mu)**2 for x in t]
    var = Mean(dev2)
    return var

def StdVar(t, mu=None):
    """标准差"""
    if mu is None:
        mu = Mean(t)
    import math
    return math.sqrt(Var(t, mu))


def Range(t):
    """极差"""
    if not t:
        return None
    return max(t) - min(t)