A sample of 67 Broad Absorption Line quasars ( BALQSOs ) from the Large Bright Quasar Survey ( LBQS ) is used to estimate the observed and intrinsic fraction of BAL quasars in optically–selected samples at intermediate ( B _ { J } \simeq 18.5 ) magnitudes .
The observed BALQSO fraction in the redshift range 1.5 \leq z \leq 3.0 is 15 \pm 3 \% .
A well–determined , empirical , k –correction , to allow for the differences in the spectral energy distributions of non–BALQSOs and BALQSOs shortward of \simeq 2100 Å in the restframe , is applied to the sample .
The result is an estimate of the intrinsic fraction of BALQSOs , in the redshift range 1.5 \leq z \leq 3.0 , of 22 \pm 4 \% .
This value is twice that commonly cited for the occurrence of BALQSOs in optically–selected samples and the figure is in reasonable agreement with that from a preliminary analysis of the SDSS Early Data Release .
The fraction of BALQSOs predicted to be present in an optical survey with flux limits equivalent to that of the FIRST Bright Quasar Survey ( FBQS ) is shown to be \simeq 20 \% .
The BALQSO fractions derived from the FBQS and the LBQS suggest that optically–bright BALQSOs are half as likely as non-BALQSOs to be detectable as S _ { 1.4 { GHz } } \gtrsim 1 mJy radio sources .