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On this page: Interpretation of C_p & C_pk C_p formula C_pk formula Prework checklist

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C_p Formula / How to Calculate C_p

Where:

$\text{LSL}$

 — Lower specification limit

$\text{USL}$

 — Upper specification limit

$s$

 — Standard deviation

C_pk Formula / How to Calulate C_pk

Where:

$\text{LSL}$

 — Lower specification limit

$\text{USL}$

 — Upper specification limit

$\bar{X}$

 — Mean

$s$

 — Standard deviation

Interpretation of C_p and C_pk:

C_p ≥ 1.3

 — fair

C_p ≥ 1.7

 — very good

C_p ≥ 2.0

 — world class

Your choice of benchmark should take into account previous best performance, whether you're using long or short-term data, and the requirements of your industry.

Prework Checklist

• Confirm customer requirements to establish the specification limits
• Collect at least 25 individual measurements, or subgroups, in timeseries.
• Ensure the process is in statistical control.
• Consider performing a measurement system analysis, if you suspect measurement error.

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Related:

• Calculator: DPMO Process Capability