Variance as function of aperture size for our source samples. Top panel: E-mode and the predictions (lines) for the BACK low-z and BACK high-z data points (solid and open circles). The crosses and stars correspond to the (noisier) signals of the four subdivisions a and b of the source samples. Bottom panel: B-mode relative to the E-mode signal (in percentage). This plot uses seven evenly spaced aperture radii instead of the eight used in the analysis.