explanation for the observed uniformity of the EW of the broad [H$\beta$]{} line in AGNs (Searle & Sargent 1968). Recall that the slopes of the Balmer line versus continuum luminosity relations (Eqs. 1 and 2) are formally steeper than unity (1.16 and 1.13 for [H$\alpha$]{} and [H$\beta$]{}, respectively). This implies that the line strength increases mildly with increasing luminosity \[EW([H$\alpha$]{}) $\propto$ [$L_{5100}$]{}$^{0.16}$; EW([H$\beta$]{}) $\propto$ [$L_{5100}$]{}$^{0.13}$\], which means that the Balmer lines show an [*inverse*]{} Baldwin effect (Baldwin 1977). While various studies have found a roughly linear relation between optical continuum luminosity and [$L_{\mathrm{H{\beta}}}$]{} (e.g., Yee 1980; Shuder 1981; Boroson & Green 1992; Dietrich [et al.]{} 2002), Croom [et al.]{} (2002), consistent with our study, report a positive slope of EW([H$\beta$]{}) $\propto$ [$L_{5100}$]{}$^{0.19}$ using composite spectra constructed from the 2dF+6dF quasar survey. These apparently conflicting results may be attributable to the different luminosity coverage in the different studies. As discussed by Croom et al., the luminosity-dependent variation of the Balmer line strengths may indicate that the shape of the ionizing continuum changes systematically with luminosity. Another possibility is that the covering factor of the line-emitting gas depends on luminosity.

The second major empirical correlation presented in this study links the line 