标准方差的计算公式是：
每一个数与这个数列的平均值的差的平方和，除以这个数列的项数，再开根号
分析：
标准方差主要和分母、分之有直接关系
这里的偏差为每一个数与平均值的差。
几个适用的理解：
1.数据分布离平均值越近，GMAT数学标准方差越小;数据分布离平均值越远，标准方差越大。
2.标准方差为0，意味着数列中每一个数都相等。
3.序列中每一个数都加上一个常数，标准方差保持不变的
4.序列中每一个数都乘以不为0的数N，标准方差扩大N倍
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