Top: χ² per pixel for joint baseline Planck, WMAP, and 408 MHz intensity analysis. Middle: confidence mask derived by smoothing the χ² map to 1deg FWHM, and thresholding at a value of . Its primary application is for the low-ℓ 2015 Planck temperature likelihood, and it is accordingly denoted LM93 (93% likelihood mask); see Planck Collaboration XI (2016) for further details. Bottom: histogram of χ² values outside the conservative PM61 mask. The grey dashed line shows the best-fit χ² distribution with a variable degree of freedom and scaling, used to account for prior and noise modelling effects; see Sect. 5.3 for further discussion.