Normalized histograms of PCNT spectral indices for different pairs of Planck frequencies. Only sources detected above the 4σ level in each pair of frequencies are considered. Sources within the Planck GAL070 mask are shown in red, whereas the sources outside the quoted Galactic mask appear in blue. Instead of the usual fixed-width bins, the histograms use optBINS, an optimal adaptive data-based binning method that adjusts itself to optimally reflect the details of the structure of the underlying distribution (Knuth 2006).