def _predict(self, fit, df):
        """
        Return a df with predictions and confidence interval

        Notes
        -----
        The df will contain the following columns:
        - 'predicted': the model output
        - 'interval_u', 'interval_l': upper and lower confidence bounds.

        The result will depend on the following attributes of self:
        confint : float (default=0.95)
            Confidence level for two-sided hypothesis
        allow_negative_predictions : bool (default=True)
            If False, correct negative predictions to zero (typically for energy consumption predictions)

        Parameters
        ----------
        fit : Statsmodels fit
        df : pandas DataFrame or None (default)
            If None, use self.df


        Returns
        -------
        df_res : pandas DataFrame
            Copy of df with additional columns 'predicted', 'interval_u' and 'interval_l'
        """

        # Add model results to data as column 'predictions'
        df_res = df.copy()
        if 'Intercept' in fit.model.exog_names:
            df_res['Intercept'] = 1.0
        df_res['predicted'] = fit.predict(df_res)
        if not self.allow_negative_predictions:
            df_res.loc[df_res['predicted'] < 0, 'predicted'] = 0

        prstd, interval_l, interval_u = wls_prediction_std(fit,
                                                           df_res[fit.model.exog_names],
                                                           alpha=1 - self.confint)
        df_res['interval_l'] = interval_l
        df_res['interval_u'] = interval_u

        if 'Intercept' in df_res:
            df_res.drop(labels=['Intercept'], axis=1, inplace=True)

        return df_res
Return a df with predictions and confidence interval

        Notes
        -----
        The df will contain the following columns:
        - 'predicted': the model output
        - 'interval_u', 'interval_l': upper and lower confidence bounds.

        The result will depend on the following attributes of self:
        confint : float (default=0.95)
            Confidence level for two-sided hypothesis
        allow_negative_predictions : bool (default=True)
            If False, correct negative predictions to zero (typically for energy consumption predictions)

        Parameters
        ----------
        fit : Statsmodels fit
        df : pandas DataFrame or None (default)
            If None, use self.df


        Returns
        -------
        df_res : pandas DataFrame
            Copy of df with additional columns 'predicted', 'interval_u' and 'interval_l'