Variability between seasons 3 and 4 at 150 GHz. Left: flux densities from 2009 versus 2010. The straight line marks the 1:1 relation while the dotted lines deviate 20% from it. Red crosses indicate highly variable sources with σ_var > 3.0; orange crosses are mildly variable sources with 3.0 > σ_var > 2.0; and gray crosses are sources that we consider non-variable. Right: fractional variation of flux density versus the mean flux density; colors represent σ_var. Other possible combinations, such as 2008-2010 or 2008-2009, yield similar results.