Comparison of HD 20 (blue circle) to the metal-poor field star compilation (gray dots) by Roederer et al. (2014) and the red horizontal branch star HD 222925 (Roederer et al. 2018b, red circle). Dark blue circles and error bars indicate the result in LTE while the light blue circles indicate the NLTE-corrected ones. In the reference samples, corrections have been applied to O I, Na I, and K I. On the abscissa we show abundances from Fe II since these are less prone to departures from the LTE assumption (Sect. 3.3).