Comparison of the probability density of ERV for short- and long-term repeats (see Tables 1 and 2) using order-sorting filter HR15N. The black line shows results for short-term repeats (i.e. pairs of observations within the same OB). The red histogram shows results for long-term repeats (i.e. spectra of the same targets but taken from different OBs where individual targets are allocated to different fibres).