In this context, the symbol \sigma always stands for the standard error of the subgroup summary statistic that is plotted on the chart. Elsewhere in this chapter, \sigma is also used to denote the standard deviation of a process, also referred to as the population standard deviation. This dual usage is standard practice.

The term standard deviation is also used by some authors to refer to this quantity; see, for example, Montgomery (1996). This chapter uses the term standard error for the dispersion of the distribution of a statistic and the term standard deviation for the dispersion of a distribution of individual measurements.

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