维基百科上提纲挈领是这么说的：Degrees of freedom (statistics)
In statistics, the number of degrees of freedom is the number of values in the final calculation of a statistic that are free to vary.
在统计学中，自由度的数量是最后统计计算中可以自由变换的变量的数量。
关于样本方差，里面提到：
i.e., the sample variance has N-1 degrees of freedom, since it is computed from N random scores minus the only 1 parameter estimated as intermediate step, which is the sample mean.
1.样本方差有N-1的自由度，因为从N个随机数减去1个参数，这1个参数是样本均值，样本均值是计算样本方差的中间一步。
2.样本方差是总体方差的无偏估计。
3.总体方差是一个事实(fact).
链接：https://www.zhihu.com/question/20983193/answer/28228799