Dependence of fractional variability index V_rms on mean flux density  ⟨ S_ν ⟩  in Ka, Q, V, and W bands for variable sources. BLLs are shown by filled circles, FSRQs by open circles, and all the other sources by plus signs. The dashed lines indicate the level of . The apparent trend of increasing variability with decreasing flux density is likely an artefact arising from the application of a significance cut in the presence of large measurement errors.