produce broad absorption line (BAL) troughs are correlated with the continuum and emission-line mechanisms. For example, quasars seen at larger inclination angles, with larger black hole masses, or with higher accretion rates may have distinct continuum and emission-line properties, and they may also be more or less likely to exhibit BAL troughs.

In the first such study, (1991) combined 34 high-ionization BALQSOs (HiBALs), 6 low-ionization BALQSOs (LoBALs) and 42 nonBALs to make composite HiBAL, LoBAL and nonBAL spectra. They used these composites to show that the emission-line properties of BALs and nonBALs are quite similar, except that the continua of LoBALs are significantly redder than the continua of HiBALs and nonBALs. & [Foltz]{} (1992) quantified this difference by determining that the reddening of the LoBAL composite compared to the HiBAL composite could be accounted for by dust reddening with a Small Magellanic Cloud (SMC; 1984; [Pei]{} 1992) reddening law with $E(B - V) = 0.1$.

More recently (2001) created composite BALQSO spectra from the FIRST Bright Quasar Survey (FBQS; 2000). Using a sample of 25 HiBALs and 18 LoBALs, they also found that BALQSOs are redder than the typical nonBALQSO and that the HiBAL and LoBAL spectra can be brought 