docs: def resid_anscombe(self, endog, mu):
          '''
          The Anscombe residuals
  
          Parameters
          ----------
          endog : array-like
              Endogenous response variable
          mu : array-like
              Fitted mean response variable
  
          Returns
          -------
          resid_anscombe : array
              The Anscombe residuals as defined below.
  
          References
          ----------
          Anscombe, FJ. (1953) "Contribution to the discussion of H. Hotelling's
              paper." Journal of the Royal Statistical Society B. 15, 229-30.
  
          Cox, DR and Snell, EJ. (1968) "A General Definition of Residuals."
              Journal of the Royal Statistical Society B. 30, 248-75.
  
          '''
          cox_snell = lambda x: (special.betainc(2/3., 2/3., x)
                                 * special.beta(2/3., 2/3.))
          return np.sqrt(self.n) * ((cox_snell(endog) - cox_snell(mu)) /
                                    (mu**(1/6.) * (1 - mu)**(1/6.)))