'(cached attribute) dffits measure for influence of an observation
based on resid_studentized_external,
uses results from leave-one-observation-out loop
It is recommended that observations with dffits large than a
threshold of 2 sqrt{k / n} where k is the number of parameters, should
be investigated.
Returns
dffits: float
dffits_threshold : float
References
`Wikipedia <http://en.wikipedia.org/wiki/DFFITS>`_'

@cache_readonly
def dffits(self):

    hii = self.hat_matrix_diag
    dffits_ = (self.resid_studentized_external * np.sqrt((hii / (1 - hii))))
    dffits_threshold = (2 * np.sqrt(((self.k_vars * 1.0) / self.nobs)))
    return (dffits_, dffits_threshold)
