Comparison of abundances taken from different studies of the same white dwarfs. The figure illustrates the range of measurements obtained due to using different observations and models. Each axis is 2 dex wide. The upper-left panel shows Si and Fe photospheric abundances for GALEX J193156.8+011745 (Vennes et al. 2010; Melis et al. 2011; Vennes et al. 2011; Gänsicke et al. 2012). The upper-right panel shows Ca and Fe photospheric abundances for GD 378 (Dupuis et al. 1993; Wolff et al. 2002; Desharnais et al. 2008; Klein et al. 2021). The bottom-left panel presents photospheric Ca and Mg abundances for PG 1015+161 (Kawka et al. 2011; Gänsicke et al. 2012; Xu et al. 2019). The bottom-right panel contains photospheric abundances of Fe and Mg for GD 40 (Shipman & Greenstein 1983; Dupuis et al. 1993; Friedrich et al. 1999; Klein et al. 2010; Jura et al. 2012).