Values of M _BH at which a Type 1 quasar survey becomes only 10% complete under a limiting magnitude of i < 20 (red solid line) and i < 24 (blue dashed line), as a function of redshift. The data points denote the posterior median values, and the error bars contain 68% of the posterior probability. The SDSS becomes highly incomplete below M _BH ~ 3 × 10 ⁸  M _☉.