Illustration of the effect of the spectral lines for determining the nitrogen abundance of star HD 207538. The thin broken lines show the normalized χ² curve when the fit is performed on individual lines (wavelength indicated in the figure). The bold solid curve shows the χ² of the analysis combining 21 N ii and N iii lines. The black dot shows the preferred value of N/H resulting from the fit of the combined lines, together with the error bars.