Spectrum of the half-difference between Sky Survey (SS) 1 and 2, (SS1 − SS2) / 2 (odd-even number surveys, shown in frame A), and (SS1 − SS3) / 2 (odd-odd, frame B), compared with the half ring spectra for the same time period at the three LFI bands. We also show the simulated errors due to gain reconstruction (Sect. 5.1). Spectra have been binned into five samples each, in order to remove high-frequency noise and highlight the general trend. At high multipoles (ℓ > 20) there is a good match between survey differences and half-rings, which means that discrepancies between survey differences are mainly due to statistical noise. Note that the biggest discrepancy between survey differences and halfrings happens for 30 GHz channels when comparing SS1 with SS2 (odd-even); this suggests that optical effects (sidelobes) are causing the discrepancy. Gain errors are calculated over the whole mission and are negligible except for the lowest multipoles of 44 GHz and 70 GHz spectra, where they probably account for the large-scale residuals found in survey difference maps.