A Control Chart for Monitoring Process Variability
نویسندگان
چکیده
The Shewhart and the Bonferroni-adjustment S control charts are usually applied to monitor the standard deviation of a quality characteristic. The control limits of these charts are constructed using approximately the normal distribution in case that the standard deviation parameter is known or unknown. In this paper, we establish a new S chart that is based approximately on the normal distribution. The control limits of the new chart are depended on both the sample group size, k, and sample subgroup size, n. Additionally, the unknown standard deviation for the proposed approach is estimated by a uniformly minimum variance unbiased estimator. This estimator has variance less than the estimators used for the Shewhart and Bonferroni approach. Meanwhile, for our proposed approach with unknown standard deviation, the out-of-control average run length is slightly less than the Shewhart approach and considerably less than Bonferroni-adjustment approach as demonstrated through Monte Carlo simulation experiments.
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