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2. Measurement Process Characterization
2.2. Statistical control of a measurement process
2.2.2. How are bias and variability controlled?

Remedial actions

Consider possible causes for out-of-control signals and take corrective long-term actions There are many possible causes of out-of-control signals.

A. Causes that do not warrant corrective action for the process (but which do require that the current measurement be discarded) are:

  1. Chance failure where the process is actually in-control
  2. Glitch in setting up or operating the measurement process
  3. Error in recording of data

B. Changes in bias can be due to:

  1. Damage to artifacts
  2. Degradation in artifacts (wear or build-up of dirt and mineral deposits)

C. Changes in long-term variability can be due to:

  1. Degradation in the instrumentation
  2. Changes in environmental conditions
  3. Effect of a new or inexperienced operator
4-step strategy for short-term An immediate strategy for dealing with out-of-control signals associated with high precision measurement processes should be pursued as follows:
Repeat measurements
  1. Repeat the measurement sequence to establish whether or not the out-of-control signal was simply a chance occurrence, glitch, or whether it flagged a permanent change or trend in the process.
Discard measurements on test items
  1. With high precision processes, for which a check standard is measured along with the test items, new values should be assigned to the test items based on new measurement data.
Check for drift
  1. Examine the patterns of recent data. If the process is gradually drifting out of control because of degradation in instrumentation or artifacts, then:

    • Instruments may need to be repaired
    • Reference artifacts may need to be recalibrated.
  1. Reestablish the process value and control limits from more recent data if the measurement process cannot be brought back into control.
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