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Sound transmission loss and sound absorption measurements are conducted to characterize acoustical performance of noise control materials and components used in vehicles. These measured data are often used to select materials and define acoustical targets. It is imperative to have accurate and repeatable data. Process variability is often monitored using measurement data collected over time. A certain amount of variability due to random causes is always expected. Acoustical measurements have inherent variability from different operators, equipment, test setup, environment etc. When variation in the measurements is due to random causes the measurements are in-control and measured data are considered “good”. However, special cause variations in the measured data such as operator error or setup error must be identified and corrected. Control chart is a popular statistical tool for monitoring process variability and improving quality.