Gas-phase metallicities of our sample galaxies as a function of stellar mass obtained from the stacked spectra. The results for galaxies in X-ray groups are shown in dark red, galaxies in non-X-ray groups are shown in orange, and field galaxies are shown in blue. The error bar indicates the measured error obtained by mock repeat measurements, while the shaded region indicates the 1σ scatter of each sample estimated from bootstrapping approaches. The black stars indicate individual measurements for X-ray group galaxies in the most massive bin available from high-quality LEGA-C observation. The cyan dashed line indicates the MZR of local z ∼ 0 SDSS galaxies obtained by Tremonti et al. (2004), while the horizontal dashed line marks the solar oxygen abundance.