Label differences between repeat observations as a function of S/N for stars with T_eff > 10 000 K. Only repeat observations carried out on different observation nights and with S/N differences of less than 20 percent are adopted. The x axis shows the mean S/N of the repeat observations. The solid lines in red show the median differences as well as the standard deviations at different S/N. The numbers marked in the figure are precision (the scatter divided by , i.e., assuming both stars contribute equally) for stars with S /N < 50, 50 < S/N < 100, and S /N > 100, respectively.