'Returns Scott\'s normal reference rule of thumb bandwidth parameter.
Notes
See p.13 in [2] for an example and discussion.  The formula for the
bandwidth is
.. math:: h = 1.06n^{-1/(4+q)}
where ``n`` is the number of observations and ``q`` is the number of
variables.'

def _normal_reference(self):

    X = np.std(self.data, axis=0)
    return ((1.06 * X) * (self.nobs ** ((-1.0) / (4 + self.data.shape[1]))))
