Top panel: histogram of measured oxygen abundances (12 + log (O/H)) derived through strong-line diagnostic ratios in blue following Sects. 2.3.3 and 2.4 and in black for all data. Bottom panel: cumulative distributions of the GRB host metallicities in different redshift intervals, where blue/red/olive colors denote redshift intervals below z = 1 (z_med = 0.6), 1 <z< 2 (z_med = 1.4), and 2 <z (z_med = 2.4), respectively. Z_⊙ is indicated by the vertical, gray dashed line. Error-regions of the 1 <z< 2 sample have a similar size to the one for z< 1, but are not shown to enhance clarity.