Fraction of likely quasars to the total number of sources with zero proper motions and parallaxes as a function of Galactic latitude b. The quasars are identified via their K-band excess (see Sect. 4.3). The fractions are shown in bins of Δb = 5 deg. The horizontal errorbars show the widths of the bins and the vertical errorbars indicate the 1σ Poisson errors for each bin. A selection efficiency of N_QSO/N_tot >  50%, using a purely astrometric identification for quasars, can only be reached at b ≳ 80 deg.