Measures of error as a function of P_rot. Blue unfilled circles plot the ‘empirical’ dispersion σ_emp, i.e., standard deviation of the measured periods, colour-coded by the number of measurements (see colour bar). Grey filled circles represent ‘formal’ errors δ_formal, as reported in the literature. Top: σ_emp versus P_rot, best (blue) and δ_formal versus reported P_rot (grey); middle: fractional error versus P_rot, where fractional error is obtained by dividing σ_emp or δ_formal (collectively termed ‘dP’) by P_rot. Bottom: same as the middle, but with the y-axis shown on a linear scale. The best-fit power laws are plotted for σ_emp (red dashed, as given in Eq. (4)) and δ_formal (purple dot-dashed, as given in Eq. (5)). The black dotted lines depict constant fractional errors of 10% and 1% for comparison. Note that the bottom panel has restricted vertical plotting range in order to show the difference in the two power laws. Points beyond the vertical range are not shown.