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Broad absorption line quasars (BALQs) present a large diversity in theirbroad absorption line (BAL) profiles. To investigate what physical parametersunderlie this diversity, we construct a sample of BALQs which coverslambda_rest ~ 800-3000 {\AA}, based on the Sloan Digital Sky Survey DR7 quasarcatalogue. The average BAL properties are evaluated by taking the ratios ofaverage BALQ spectra to the average spectra of matched samples of non-BALQs,where the matching is based on various emission properties. We find thefollowing properties. (i) There is no detectable Lyman edge associated with theBAL absorbing gas (tau<0.1). (ii) The known increase of average absorptiondepth with the ionization potential extends to the higher ionization N V and OVI BALs. We also find that the C IV BAL profile is controlled by twoparameters. (i) The He II emission EW, which controls the typical velocity ofthe C IV BAL, but does not affect the absorption depth. (ii) The spectral slopein the 1700-3000 {\AA} range (alpha_UVl), which controls the C IV peakabsorption depth, but does not affect the typical velocity. The He II EW andalpha_UVl also control the observed fraction of quasars that are BALQs. Wesuggest that a lower He II EW may indicate a weaker ionizing continuum, whichallows the outflow to reach higher velocities before being overionized,possibly without a need to invoke a shielding gas. A redder continuum mayindicate a more inclined system, and a larger covering factor and larger columnof the outflow along the line of sight.

The average absorption properties of broad absorption line quasars at 800 < lambda_rest < 3000 {\AA}, and the underlying physical parameters