Mean parallax for KiDS DR4 QSO candidates as a function of minimum classification probability. The Gaia observations have a global mean offset, which is imprinted in the QSO mean parallax distribution. The offset for SDSS spectroscopic QSOs equals −0.017 ± 0.001 mas (standard error on the mean). We calculated the acceptable offset based on star and galaxy contamination estimated in the experiments. It equals −0.01 ± 0.0015 mas.