我是Python的新手，我想知道为什么np.var（x）与np.cov（x，y）输出中的cov（x，x）值给出不同的答案。他们不应该一样吗？我知道它与偏见或ddof有关，与规范化有关，但是我不确定这意味着什么，也找不到任何能专门回答我问题的资源。希望有人能帮忙！
最佳答案
在numpy中，cov的默认“ delta自由度”为1，而var的默认ddof为0。从注释到numpy.var
Notes
-----
The variance is the average of the squared deviations from the mean,
i.e., ``var = mean(abs(x - x.mean())**2)``.
The mean is normally calculated as ``x.sum() / N``, where ``N = len(x)``.
If, however, `ddof` is specified, the divisor ``N - ddof`` is used
instead. In standard statistical practice, ``ddof=1`` provides an
unbiased estimator of the variance of a hypothetical infinite population.
``ddof=0`` provides a maximum likelihood estimate of the variance for
normally distributed variables.
因此，您可以通过以下方式让他们同意：
In [69]: cov(x,x)#defaulting to ddof=1
Out[69]:
array([[ 0.5, 0.5],
[ 0.5, 0.5]])
In [70]: x.var(ddof=1)
Out[70]: 0.5
In [71]: cov(x,x,ddof=0)
Out[71]:
array([[ 0.25, 0.25],
[ 0.25, 0.25]])
In [72]: x.var()#defaulting to ddof=0
Out[72]: 0.25