Hebrew University, Jerusalem
I will present a measurement of the probability distribution function (PDF) of the Lya transmitted flux at redshift 2<z<4 from a sample of SDSS quasars. The PDF is obtained by fitting a powerlaw continuum on a spectrum-by-spectrum basis. The SDSS data are compared to a theoretical PDF drawn from mock lognormal spectra, whose statistical properties have been constrained to match the observed Lya flux PDF and power spectrum of Keck spectra. The lognormal model fits the Keck data very well at all redshifts. However, it produces a poor fit to the SDSS data, unless large continuum correction are applied. A decrease of 15 per cent in the mean quasar continuum with a rms variance at the 20 per cent level greatly improves the fit. This strongly suggests that there is a break in the spectral slope of high-redshift quasars around the Lya emission line.