Difference between auto and cross-spectra for the 100 GHz half-mission maps, divided by the noise estimate from half-ring difference maps (blue and green lines). Noise estimates derived from half-ring difference maps are biased low. We fit the average of both half-mission curves (black line) with a power law model (red line). The analysis procedure is applied to the Stokes parameter maps I, Q, and U (top to bottom). All data power spectra are smoothed.