Return unbiased variance over requested axis.
Normalized by N-1 by default. This can be changed using the ddof argument
Exclude NA/null values. If an entire row/column is NA, the result
will be NA.
If the axis is a MultiIndex (hierarchical), count along a
particular level, collapsing into a scalar.
Delta Degrees of Freedom. The divisor used in calculations is N - ddof,
where N represents the number of elements.
Include only float, int, boolean columns. If None, will attempt to use
everything, then use only numeric data. Not implemented for Series.
To have the same behaviour as numpy.std, use ddof=0 (instead of the