Residuals (ΔEW = EW_calc–EW_obs) from the calibrations of index 38 as a function of, from top to bottom, EW_obs, T_eff, [Fe/H], and log g. The horizontal black solid lines indicate the perfect agreement. The blue and red dashed lines show, respectively, the 1σ and 2σ limits, where σ = 18.36 mÅ  is the standard deviation of the residuals (see Section 4). The green dotted line in the upper panel represents the linear fit to the residuals, which has a correlation coefficient R² = 0.007. In the panels for T_eff, [Fe/H], and log g, the fit cannot be distinguished from the black line. No significant trends can be observed for the residuals as a function of the EWs and atmospheric parameters.