code: def _create_results_summary(self):
          
          # Make sure we have all attributes needed to create the results summary
          needed_attributes = ["params",
                               "standard_errors",
                               "tvalues",
                               "pvalues",
                               "robust_std_errs",
                               "robust_t_stats",
                               "robust_p_vals"]
          try:
              assert all([hasattr(self, attr) for attr in needed_attributes])
              assert all([isinstance(getattr(self, attr), pd.Series)
                          for attr in needed_attributes])
          except AssertionError:
              msg = "Call this function only after setting/calculating all other"
              msg_2 = " estimation results attributes"
              raise NotImplementedError(msg + msg_2)
  
          self.summary = pd.concat((self.params,
                                    self.standard_errors,
                                    self.tvalues,
                                    self.pvalues,
                                    self.robust_std_errs,
                                    self.robust_t_stats,
                                    self.robust_p_vals), axis=1)
  
          return None
docs: Create the dataframe that displays the estimation results, and store
          it on the model instance.
  
          Returns
          -------
          None.