RMSEs (residuals) and predictive uncertainties for RV and DS obtained using the HO and CV approaches across the ten different test sets. First row: HO model showing the RMSE between the predicted and true values. Second row: HO model showing the mean predictive uncertainties, defined as the standard deviation over 100 MC-DO. Third row: CV model showing the RMSE obtained with the five-fold CV approach. Fourth row: CV model showing the corresponding mean predictive uncertainties.