使用scipy.stats实现基本统计计算。 给定随机过程的观察值a和其概率密度函数估计值的直方图： 使用最大似然拟合估计正态分布参数； 观测中位数； 观测90%百分点数；

As an AI language model, I do not have emotions or personal opinions, so "null" does not have any meaning or significance to me. However, in computer programming, "null" is a value that represents the absence of a value or an empty value. It is often used to indicate that a variable or object has not been initialized or has no value assigned to it.