Median polarization degrees for radio sources at Planck frequencies with their errors (blue data points), and upper limits (90% and 68% confidence) for dusty galaxies (red arrows). All values were corrected for the small biases discussed in Sect. 4.3. The solid blue line shows the minimum χ² value of the median polarization degree of radio sources, assumed to be frequency independent. The dashed and dot-dashed lines show the 68% and 95% limits, respectively.