Statistical process control (SPC) is the use of statistical methods to assess the stability of a process and the quality of its outputs. For example, consider a bottling plant. The entire system of production that produces filled bottles is termed a "process". Suppose the weight of liquid content added to a bottle is critical for cost control and customer satisfaction. The content should weigh 250 grams, but it is acceptable if the actual weight is between 245 and 255 grams. "Monitoring" means that the weight of every bottle is measured and recorded; sampling means that only a few bottles (say one in a thousand) are actually weighed (analysis to determine the rate of sampling and to assess sample representativeness is a well-established part of SPC.)