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This paper presents recent progress in reducing the measurement uncertainty for crystalline silicon (c-Si) and thin-film PV modules. It describes the measurement procedure and the uncertainty analysis, as applied at the CalLab PV Modules, Fraunhofer ISE's laboratory for module measurements. The uncertainty analysis covers the complete calibration process in detail, including measurements, correction to standard testing conditions, and determination of electrical module parameters (ISC, PMPP, VOC, etc.) from the I-V curve. Differences between c-Si and thin-film modules are addressed, most importantly in terms of spectral mismatch factor and short timescale stability problems. The paper outlines the importance of a comprehensive quality assurance system in a calibration laboratory as a prerequisite for accurate measurements on a daily basis. Particular attention is paid to results from a series of measurements taken every three weeks over a three-year period and conducted as part of the quality assurance system. In conclusion, this paper introduces a best-case uncertainty for c-Si module calibration of 1.6% for PMPP and 1.3% for ISC. This represents the lowest reported uncertainty for full size module calibration in a laboratory so far. The presented uncertainty in PMPP of cadmium telluride and single-junction amorphous silicon modules is 2.9% and 1.8%, respectively. All mentioned uncertainties are expanded uncertainties (k = 2).