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PV modules are primarily rated at standard test conditions (STC), which corresponds to 1000 W/m2 irradiance, 25°C cell temperature, and AM 1.5 spectrum. This paper presents the statistical analyses of efficiency data obtained for over 300 crystalline silicon modules from various manufacturers between 1999 and 2008. Modules were tested at Arizona State University. The objective is (1) to show how the rated efficiencies compare to the measured efficiencies (of manufacturers' supplied modules to a certification laboratory) at STC over time, and (2) to compare measured efficiencies under two other different test conditions: SOC (standard operating condition) and Low E (low irradiance). This paper presents three major findings related to: rated versus measured STC data on manufacturers' supplied modules to a certification laboratory; performance variability over time; and statistical significance of the efficiency increase over the time period considered.