=+Control charts are constructed by taking successive samples from the output of a process, making measurements on the sampled items, and then plotting summary statistics of these results. Figure 6.4 shows a typical control chart. The samples, also called subgroups, of size n are taken at regular intervals of time. For each subgroup, a summary statistic is calculated and plotted (on the vertical axis) versus the subgroup number (on the horizontal axis). Any statistic of interest can be calculated, but the most commonly