Top: relative difference between the global τ_HB and the τ_HB index only based on ground-based data versus the global index. Note that in the estimate of the global index the priority in selecting stars was given to space-based (ACS at HST) data. Bottom: as the top panel, but the relative difference is between the global index and the τ_HB index only based on space-based data. The standard deviation of the estimates is represented by the error bar at the bottom left corner of the panels. At the top right corners we display the mean relative difference.