Relative variation of σ against the mean S/N of the spectra. The four different symbols show the values relative to the four different bootstrap routines described in Sect. 4.4, and the points are colour-coded by the S/N of the spectra to which they refer. The uncertainties are about 10% for the lowest S/N spectra (red) and ~4% for the highest S/N spectra (blue). The colour gradient of the data points allows distinguishing the different systems.