by Wu et al. (2004), who converted [$L_{5100}$]{} to [$L_{\mathrm{H{\beta}}}$]{}, but their analysis was confined to the original reverberation mapping sample of Kaspi et al. (2000), which consists, by necessity, of AGNs that are variable on timescales probed by current reverberation mapping experiments. This selection effect may result in subtle biases, and, in any case, the sample suffers from limited statistics. Secondly, we substitute the line width of [H$\beta$]{} with that of [H$\alpha$]{}, using a newly derived empirical correlation between the two.

Throughout we assume the following cosmological parameters to calculate distances: $H_0 = 100h = 71$ [km s$^{-1}$]{} Mpc$^{-1}$, $\Omega_{\rm m} = 0.27$, and $\Omega_{\Lambda} = 0.75$ (Spergel [et al.]{} 2003).

+5mm

The Sample
==========

We utilize the large and homogeneous database provided by the Sloan Digital Sky Survey (SDSS; York [et al.]{} 2000), which will eventually obtain imaging and follow-up spectroscopy for about one-quarter of the sky. Briefly, spectroscopic candidates are selected based on multi-band imaging (Strauss [et al.]{} 2002) with a drift-scan camera (Gunn [et al.]{} 1998). A pair of spectrographs is fed by $3\arcsec$-diameter fibers, covering $\sim 3800 - 9200$ Å with an instrumental resolution of $\lambda/\Delta \lambda \approx 1800$ (Gaussian $\sigma_{\rm inst} \approx 71$ [km 