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This International Standard specifies procedures for the statistical control of processes by using control charts based on calculating the arithmetic average of a sample and using warning limits and action limits. It is assumed that for a large lots and for the mass output of piece and batch production, such a measure of quality is a random variable following a normal distribution. However, when averages of four or more items are plotted, this assumtion of a normal distribution is not necessary for cantrol purposes (see 4.2).