An example of a BOSS quasar spectrum of redshift 3.239. The red and blue lines cover the forest region used here, 104.5 < λ_rf < 118.0. This region is sandwiched between the quasar’s Lyβ and Lyα emission lines respectively at 435 and 515 nm. The blue line is an estimate of the continuum (unabsorbed flux) by method 2 and the red line is the estimate of the product of the continuum and the mean absorption by method 1.