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.