Which of the following measures is increased when process performance is improved?

Question

Options

• AVariability range
• BCapability index
• CRepeatability index
• DSpecification limits

• A
  3% (1)
• B
  94% (29)
• C
  3% (1)

Explanation

Capability index (Cp or Cpk) directly quantifies how well a process fits within specification limits, so when performance improves - through reduced variation or better centering - the index value rises. A Cpk of 1.33 is considered capable; improving the process pushes that number higher.

Why the distractors are wrong:

• A (Variability range): Improved processes produce less variation, so the variability range decreases, not increases.
• C (Repeatability index): Repeatability belongs to measurement system analysis (Gauge R&R), not process performance improvement - it measures instrument/operator consistency, not process output.
• D (Specification limits): Spec limits are set by the customer or design team, not by process performance. They stay fixed regardless of how well you improve the process.

Memory tip: Think of Cpk as a "process report card" - just like a student's GPA goes up when they perform better, the capability index goes up when the process performs better.

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