Modified Simple Robust Control Chart Based on Median Absolute Deviation
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Date
2012
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Abstract
The control limits derived for the Median Absolute Deviation (MAD) based Standard deviation (S) control chart
proposed by Abu-Shawiesh was for monitoring quality characteristics when a standard value of sigma (σ) is known or given by the management/ engineers. When sigma (σ) is unknown and we are interested in monitoring past/nonnormal data, then there is the need to modify the simple robust control limits. In this paper, the control limits for the Shewhart ¯X and S control charts based on median absolute deviation were modified using the concept of three sigma (3σ) limits. An evaluation performance tool was also developed to evaluate the efficiency of the modified control chart. An algorithm implemented on the S-Plus programming language was developed to compute the two evaluation parameters used in this study. The results show that the control limits interval and the average run
length for the modified control charts are smaller than that of the existing control charts. Therefore, the modified
control limits are more efficient than the existing control limits. It is recommended that the modified control limits
be used when monitoring past/non-normal data or when there is no standard value of sigma specified by the process engineer/ management.
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Keywords
Median Absolute Deviation (MAD), Average Run Length (ARL),, Control Limits Interval (CLI), Process Control, sigma approach