Purpose:To propose a phase I control chart that is distribution free when process is in-control (IC).Summary:The proposed method can be used to define the IC state of the process variability and to aid in identifying an IC reference sample. The proposed scale charting method is compared with the traditional R- and S-charts using Monte Carlo simulation. It is shown that the IC performance of R- and S-charts is poor in phase I, both under normal and non-normal cases. The proposed method achieves desired IC performance in both the cases and is sensitive to subgroup differences in process scale. A recommendation is given for combining this method with an existing distribution free phase I chart for process location. (23 refs.)
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