Analysis of the empirical uncertainty for short-term repeat observations of RV using filter HR15N. The solid line in plot a) shows the variation of ERV × (S/N)/(1 + ( [ vsini ] /C)2)3/4 with S/N. The horizontal line indicates the value of parameter B in Eq. (1) fitted to the full dataset. Blue crosses show the estimated values B as a function of S/N corrected for the measured variation of B with Teff (see Sect. 4.1 and Table 3). Plot b) shows the variation of ERV × S/N with vsini. The solid line show the relationship predicted using the theoretical value of C and the value of B from plot a). The dashed line shows a curve of similar functional form using parameters B and C fitted to the binned data. In plots a) and b) the y-axis shows an estimate of the standard deviation based on the MAD divided by 0.72 (see Sect. 3.2). Plot c) shows the cumulative probability distribution (CDF) of the normalised uncertainty in RV for short-term repeats. The red solid line shows results for measured data, the dashed line shows the cumulative distribution of a Gaussian with unit dispersion, and the diamond symbols show the cumulative distribution function for a Student’s t-distribution with ν = 6.