W_LAB/W_GBT for every spectrum that could be compared (dots, color-coded by region). Filled circles show, for each of the observed regions, the weighted average ratio and its uncertainty (smaller than the filled circle for the typical uncertainty of  ~0.004 in the regional averages), along with the region’s average W_GBT and its dispersion (indicated by the horizontal errorbars). These comparisons suggest that a single scale factor (horizontal solid line, from Eq. (9)) might be too simplistic.