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Figure 9: Fraction of obscured AGN as a function of redshift. Upper panel: direct measurements using the sources on the ECDF-S field only (black circles with dashed error bars), from the sample of Treister and Urry ([31]; gray circles) and combining both samples (black circles with solid error bars). The expected observed fraction for an intrinsic fraction of 3 : 1 obscured to unobscured AGN, accounting for optical and X-ray selection effects, is shown by the black solid line. As can be seen, while the observed fraction of obscured AGN declines toward higher redshifts, if the X-ray and optical selection effects and the luminosity dependence of the obscured AGN fraction are taken into account, this decline should be even stronger. Bottom panel: Inferred fraction of obscured AGN relative to an intrinsically constant fraction after correcting for selection effect and including the luminosity dependence of the obscured AGN fraction. Symbols are the same as for the upper panel. The corrected fraction of obscured AGN increases with redshift following a power law of the form with .