def summary(self):
        """
        Summary statistics describing the fit.

        Returns
        -------
        df : pd.DataFrame
            Contains columns coef, se(coef), lower, upper

        See Also
        --------
        ``print_summary``
        """
        df = pd.DataFrame(index=self.params_.index)
        df["coef"] = self.params_
        df["se(coef)"] = self.standard_errors_
        df["lower 95% bound"] = self.confidence_intervals_["lower 95% bound"]
        df["upper 95% bound"] = self.confidence_intervals_["upper 95% bound"]
        return df
Summary statistics describing the fit.

        Returns
        -------
        df : pd.DataFrame
            Contains columns coef, se(coef), lower, upper

        See Also
        --------
        ``print_summary``