spectrum could yield $V$ and also $R$, through measurement of $L$ [e.g., @Wandel99; @McLure02; @Vestergaard02; @Corbett03; @Vestergaard04; @Kollmeier06; @Vestergaard06; @Fine08; @Shen08a; @Shen08b; @Vestergaard08]. Of the three strong emission lines generally used to estimate central black hole masses, the $R$–$L$ relationship is only well-established for $\lambda4861$ [@Bentz13 and references therein, but see the discussion in §\[section:hbpredictor\]]. Empirically establishing the $R$–$L$ relationship for $\lambda2798$ [@Homayouni20] and $\lambda1549$ [@Peterson05; @Kaspi07; @Trevese14; @Lira18; @Grier19; @Hoormann19] has been difficult.

Masses based on the $\lambda1549$ emission line, in particular, have been somewhat controversial. Some studies claim that there is good agreement between masses based on  and those measured from other lines [@Vestergaard06; @Greene10b; @Assef11]. On the other hand, there are several claims that there is inadequate agreement with masses based on other emission lines [@Baskin05; @Netzer07; @Sulentic07; @Shen08b; @Shen12; @Trak12]. [@Denney09a] and [@Denney13], however, note that there are a number of biases that can adversely affect single-epoch mass estimates, with low $S/N$ “survey quality” data being an important problem with some of the studies for which poor agreement between  and other lines is found. It has also been argued, however, that some fitting methodologies are more affected by this than others [@Shen19]. There have been more 