2.
Measurement Process Characterization
2.2.
Statistical control of a measurement process
2.2.2.
How are bias and variability controlled?
2.2.2.4.
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Remedial actions
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Consider possible causes for out-of-control signals and take
corrective long-term actions
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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:
- Chance failure where the process is actually in-control
- Glitch in setting up or operating the measurement process
- Error in recording of data
B. Changes in bias can be due to:
- Damage to artifacts
- Degradation in artifacts (wear or build-up of dirt and mineral
deposits)
C. Changes in long-term variability can be due to:
- Degradation in the instrumentation
- Changes in environmental conditions
- Effect of a new or inexperienced operator
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4-step strategy for short-term
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An immediate strategy for dealing with out-of-control signals associated
with high precision measurement processes should be pursued as follows:
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Repeat measurements
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- 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.
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Discard measurements on test items
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- 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.
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Check for drift
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- 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.
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Reevaluate
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- 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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