Evolution of the fraction of CTK sources over the total AGN population across redshift. Our results are shown with the red shaded regions, which represent the 1σ (darker red) and 2σ (lighter red) confidence regions. The grey-hatched area corresponds to the Buchner et al. (2015) results (90% confidence interval) for AGNs in the 43.2 ≤ log L_X ≤ 43.6 interval. The blue-hatched area shows the Aird et al. (2015b) estimates (99% confidence interval) for sources with log L_X = 43.5. The green squares show the Laloux et al. (2023) results; quoted upper limits are 3σ. Pink triangles correspond to the results by Lanzuisi et al. (2018) and purple diamond corresponds to a study by Masini et al. (2018). The black (Georgantopoulos et al. 2025) and yellow (Burlon et al. 2011) circles show the CTK fraction in the local Universe using Swift/BAT selected AGNs. Red diamonds show the fraction of CTK sources estimated through the integration of the Ananna et al. (2019) luminosity function. Open symbols show the result in the range (24 ≤ log N_H < 26), while filled symbols are for (24 ≤ log N_H < 25).