Footnotes

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.