Average wavelength differences between HFR and experiment (in mÅ), , for K lines of ions belonging to three isoelectronic sequences as function of the effective charge, Z_c = Z − N + 1. Full squares: Be-like sequence (N = 4); open diamonds: O-like sequence (N = 8); full circles: F-like sequence (N = 9). Experimental data for ions with Z_c ≤ 15 are taken from the laser-produced plasma (Faenov et al. 1994) and plasma focus discharge in argon (Biémont et al. 2000) experiments. EBIT measurements (Beiersdorfer et al. 1991; Smith et al. 1995; Decaux et al. 1997) are considered for ions with Z_c > 15. Experimental error bars are included.