算方差计算公式应该是这个吗？

问题如下：

An investment has probabilities of 0.1, 0.3, 0.2, 0.3, and 0.1 of giving one-year returns equal of 30%, 20%, 10%, 0%, and −10%. What is the mean return and the standard deviation of the return?

解释：

The mean return is 10%. The expected squared return is 0.024 so that the standard deviation of the return is (0.024-0.01)^(1/2) = (0.014)^0.5 = 0.118 or 11.8%.

有一个投资产品获得30%回报率的可能性是0.1，20%回报率的可能性是0.3，10%回报率的可能性是0.2,0%回报率的可能性是0.3，-10%回报率的可能性是0.1。

求回报率的均值与标准差

均值=0.1*30%+0.3*20%+0.2*10%+0.3* 0%+0.1*(-10%)=0.1=10%

expected squared return=0.1*(30%)^2+0.3*(20%)^2+0.2*(10%)^2+0.3*(0%)^2+0.1*(-10%)^2=0.024

标准差=(0.024-01*0.1)^(1/2)=0.118 or 11.8%.