方差（关于均值） 方差 \(\sigma^2\) 量化了与期望均值 \(E[x]\) 的预期距离（平方）。样本方差 \(S^2\) 量化了与样本均值的实际距离（平方）。开方后得到标准差 \(\sigma\) 或 \(S\)。在一次考试后，我...

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