Constraints on the faint-end slope β and the break magnitude M* ₁₄₅₀. The shaded region shows the parameter space that is consistent with the inferred 1σ confidence upper limits on the quasar space density at z ~ 5 derived in our study. A dot and triangle are the results inferred at z ~ 4 by Ikeda et al. (2011) and Glikman et al. (2011), respectively. Note that we estimate the errors of M* ₁₄₅₀ and the faint-end slope by applying a weighted least-squares method, while Glikman et al. (2011) estimated them by applying the STY maximum-likelihood method (Efstathiou et al. 1988).