The concurrent realization of multiple polarization-based link impairments such as polarization-mode dispersion, polarization-dependent loss, and differential-attenuation slope creates a nontrivial measurement and analysis problem. The difficulties concerning the robustness of raw data, of minimizing drift artifacts experienced during multiple wavelength measurements, and the analysis methods that lead to physical significant interpretations are addressed. Measurements of an in-service wavelength-division-multiplexed metro-area network are presented that explicitly illustrate the limitations when using industry-standard commercial test equipment.