Bias parameter as a function of redshift for various X-ray AGN selected samples (see the text for details). Gray triangles represent luminosity-dependent subsamples of the main AGN sample used here. The gray filled ellipses indicate optical AGN clustering measurements (left to right: Coil et al. 2007; Ross et al. 2009; Porciani et al. 2004). The dotted lines show the expected b( z) of typical DMH masses M _DMH based on Sheth et al. (2001). The masses are given in log M _DMH in units of h ⁻¹  M _☉.