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A Control Chart Based on Sample Median for the Detection of a Permanent Shift in the Process Mean

Summary: [This abstract is based on the author's abstract.]The X-bar chart is frequently used to monitor the process mean quality level of a quality characteristic. However, the sample average, or X-bar, is sensitive to outliers that may be due to the presence of assignable causes or common causes, whereas the median statistic is robust to outliers. When past experience of a process indicates the presence of outliers due solely to common causes, the median chart is the best choice because it ignores these outlying points and detects only permanent shifts in the process, hence producing fewer false out-of-control signals than the X-bar chart. Additionally, the median chart is an easy-to-plot chart, especially for odd sample sizes.

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  • Topics: Sampling
  • Keywords: Assignable cause,Common causes,Control limits,In-control process,Out-of-control process,Outliers,Robust design,Sample estimates,Sampling,X-bar control charts
  • Author: Khoo, Michael B.
  • Journal: Quality Engineering