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 (BJ ≃ 18.5) magnitudes. The observed BALQSO fraction in the redshift range 1.5 ≤ z ≤ 3.0 is 15% ± 3%. A well-determined empirical k-correction, to allow for the differences in the spectral energy distributions of non-BALQSOs and BALQSOs shortward of ≃2100 Å in the rest frame, is applied to the sample. The result is an estimate of the intrinsic fraction of BALQSOs of 22% ± 4% for the redshift range 1.5 ≤ z ≤ 3.0. 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 ≃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 S1.4 GHz ≳ 1 mJy radio sources.

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