Function variance 

pub fn variance<T>(
    data: &[T],
    dims: [usize; 3],
    ddof: usize,
) -> Vec<<T as Scalar>::Real>
where
    T: Scalar,
    Sum: Reduction<T, Total = T>,

Two-pass variances, real for complex data, with denominator len - ddof. Empty lanes and nonpositive denominators give NaN.