The distinction between ‘‘Phase I’’and ‘‘Phase II’’control charts is fairly recent in the statistical process control literature. The classic presentation of control charts given in many textbooks blurs this distinction. Phase II control charts treat the control chart parameters as known. Phase I control charts use a ‘‘base period,’’which is the period used to estimate the control chart parameters. One legitimately may view a Phase II control chart as a sequence of hypothesis tests. Phase I control charts are much more an exploratory data analytic tool. In actual practice, the Phase II control limits are based on the results from the Phase I study. Of course, the better we estimate the control chart parameters in Phase I. the better the Phase II control chart will perform. Phase I control charts do not have any previous information upon which the user can derive appropriate control limits. For example, consider standard X and R charts. The appropriate control limits for the X charts are