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tokens: 2. Data sets and Mg <sc>ii</sc> absorber detection 2.1. X-shooter spectral samples The GRB afterglow spectra were collected in multiple semesters between 2009 and 2016 taking advantage of the X-shooter guaranteed program and subsequent open time proposals. With a uniform set of selection criteria for target-of-opportunity follow-up adopting the criteria for the low-resolution study of Fynbo et al. (2009) , that is, mainly avoiding regions with high Galactic extinction and bursts near to the Sun, the total number of obtained afterglow spectra is 116 until October, 2016. All spectra were obtained with VLT/X-shooter, which provides intermediate spectral resolutions of measured from UV to near-IR wavelengths. The S/N of the spectra vary as some afterglow spectra were obtained several hours to a few days after the GRB trigger, when only host spectral signatures and no characteristic power-law afterglow signature was visible. Removing these from the sample leaves 81 afterglow spectra, again with varying S/N between 1 and 50 per pixel for different bursts and wavelength coverage (see Selsing et al., in prep., for details). The data were reduced with the ESO VLT/X-shooter pipeline ( Modigliani et al. 2010 ) managed by Reflex ( Freudling et al. 2013 ), and with our own post-processing steps to improve the flux calibration and rejection of bad pixels (Selsing et al., in prep.) with reduction scripts made available online 2 . The redshifts of the GRBs lie in the range . Turning to the quasar sample, the XQ-100 survey is a VLT/X-shooter ESO Large Programme that targeted 100 quasars at with a S/N higher than 20 per pixel over the entire wavelength range (see Lopez et al. 2016 , for details). The quasars were selected without prior knowledge of the presence of strong absorbers along our line of sight. The quasars and a large majority of GRB afterglows were observed with X-shooter slit widths of 1.0, 0.9, and 0.9 arcsec for the UVB, VIS, and NIR-arms, respectively, resulting in similar spectral resolutions between the two samples. The redshift distributions of the GRB and quasar samples are shown in Fig. 1 . By construction the quasars have a quite distinct distribution as high-redshift objects were selected, whereas there are no intrinsic constraints for the GRB redshift selection. To match the redshift distribution of GRBs we could include other quasars observed with X-shooter, using archive data. However, these quasars might have been selected on the basis of the presence or lack of any kind of intervening and intrinsic absorption line systems, so we do not expand the quasar sample beyond the XQ-100 data. Fig. 1 Redshift distributions for the two background source samples (XQ-100 and XSGRB). The 81 GRB afterglows and 100 quasars have median redshifts of and , respectively. 2.2. Additional data sets Since the earlier discrepancy on the incidence of Mg ii absorbers relied on the statistics of GRB absorbers obtained to that date, we also examined another