The variance of a random variable measures how far from the expected value its values typically are.
随机变量的方差衡量随机变量值与期望值偏离的程度。
Expected value is good and variance is bad because that's risk; that's uncertainty.
期望值越高越好,方差就相反,因为方差代表着风险,也就是不确定性
We want a high expected value of returns, but we don't like variance.
我们希望收益的期望值较高,并且稳定
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