'Computes the test statistic.  See p.371 in [8].'

def _compute_test_stat(self, Y, X):

    lam = self._compute_lambda(Y, X)
    t = lam
    if self.pivot:
        se_lam = self._compute_se_lambda(Y, X)
        t = (lam / float(se_lam))
    return t
