Efficiency metric (E_tot: the ratio of total number of sources in the selection to good known variables in WVSC1) using Eq. (16) for s = 2 (black lines) and for s = 3 (grey lines). Solid lines denote E_WVSC1, the fraction of the good variables in the selection, while the dashed lines denote E_tot. A good choice of α returns a high fraction of good variables E_WVSC1 for a low value of the efficiency metric E_tot.