dispersion  is more physically intuitive, but it is sensitive to the line wings, which are often badly blended with other features. All three of the strong lines usually used to estimate masses — $\lambda4861$, $\lambda2798$, and $\lambda1549$ — are blended with other features: the $\lambda4570$ and $\lambda\lambda$5190, 5320 complexes [@Phillips78] and $\lambda4686$ in the case of , the UV  complexes in the case of , and $\,\lambda1640$ in the case of . The FWHM can usually be measured more precisely than  (although @Peterson04 note that the opposite is true for the rms spectra, described below, which are sometimes quite noisy), but it is not clear that FWHM yields more [*accurate*]{} mass measurements. In practice, FWHM is used more often than  because it is relatively simple to measure and can be measured more precisely while  often requires deblending or modeling the emission features, which does not necessarily yield unambiguous results.

There are, however, a number of reasons to prefer  to FWHM as the line-width measure for estimating AGN black hole masses. [@Fromerth00] point out that  better characterizes an arbitrary or irregular line profile. [@Peterson04] note that  produces a tighter virial relationship than FWHM, and [@Denney13] find better agreement between -based and 