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Keywords:

  • capability index;
  • likelihood;
  • predictive inference;
  • simulation;
  • statistical process control

Summary

When monitoring highly capable processes, it is often desirable to tolerate small instabilities in order to avoid tempering. One approach in this setting is to monitor the capability of the process dynamically and signal if the estimated capability reaches an unacceptably low level. We suggest that monitoring the probability of the next item not falling between the specification limits is a more natural scale to evaluate risk, and offers appreciable flexibility. We use a statistical model and a window of data to evaluate this probability and decide if the process should be halted immediately based on that estimate. The properties of this method are explored numerically and a case study is provided.