Redshift distributions of galaxies (top) and QSOs (bottom) that appear as candidates of different classes based on variability criteria. The galaxies with QSO-like high variability are typically found at high redshifts of ~0.5, and those with star-like low variability have lower redshifts than other galaxies. SDSS QSOs are dominated by QSO-like variability except at z < 0.3. The bins of each diagram are normalised with respect to the total samples of spectroscopic SDSS galaxies and QSOs, respectively.