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Statistical process control (SPC) is a mathematical technique using charts analyze a production process. The analysis helps systematically fine tune the process. The approach Granite Rock Company, of Watsonville, Calif., takes for evaluating production is to track aggregate gradations as they reflect on the relative screening efficiency. There are essentially three phases to go through when first applying statistical process control: determining how to use SPC; convincing employees to accept SPC and getting everyone to use the information; and fine tuning--this includes the variability associated with aggregate gradations. Granite Rock uses SPC to monitor aggregate gradation. SPC's goal is to take massive amounts of numerical data and analyze, interpret, and present it in an understandable visual form (such as a chart or graph), making it easier to detect trends. By forcing categorization, SPC creates evidence demonstrating how consistent products are. It helps project what the aggregate gradation will be before it is shipped.