Box-and-whisker plots for the absolute deviation between MR-MP valence level energies and the recommended values in the NIST online database (Kramida et al. 2018) for the L-shell ions Fe. The number of level energies included in the statistical analysis is given on top of each box. Energy levels that are potential outliers, i.e., those with deviation lower than −1.0 eV and higher than 0.6 eV, are not included in the plots.