Top panel: normalized histogram of the signal-to-noise values in the LABOCA maps at the 24 μm positions (dash-dotted line), compared with the mean histogram from 20 sets of random positions (solid line). Lower panel: difference signal between the signal-to-noise histogram above, showing that the main part of the stacked signal is due to sources with signal-to-noise ratios between 0.5 and 2. This histogram clearly shows a deficit of points at negative signal-to-noise units and an excess at positive signal-to-noise units.