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We performed a search for WLQs in the spectroscopic data from the SloanDigital Sky Survey Data Release 7 based on Kohonen self-organising maps fornearly 10^5 quasar spectra. The final sample consists of 365 quasars andincludes in particular a subsample of 46 WLQs with equivalent widths W(MgII) <11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with thoseof control samples of ordinary quasars. Particular attention was paid toselection effects. The WLQs have, on average, significantly higherluminosities, Eddington ratios, and accretion rates. About half of the excesscomes from a selection bias, but an intrinsic excess remains probably causedprimarily by higher accretion rates. The spectral energy distribution shows abluer continuum at rest-frame wavelengths > 1500 A. The variability in theoptical and UV is relatively low, even taking the variability-luminosityanti-correlation into account. The percentage of radio detected quasars and ofcore-dominant radio sources is significantly higher than for the controlsample, whereas the mean radio-loudness is lower. The properties of our WLQsample can be consistently understood assuming that it consists of a mix ofquasars at the beginning of a stage of increased accretion activity and ofbeamed radio-quiet quasars. (Abstract modified to match the arXiv format)

A large sample of Kohonen-selected SDSS quasars with weak emission lines: selection effects and statistical properties