Cp (Process Capability) is a measure of how well a process can produce output within given specification limits, assuming it is perfectly centered. It compares the spread of the process variation (6σ) to the allowable tolerance range (USL – LSL).
Cp (Process Capability):
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Measures the potential capability of a process assuming it is centered within the specification limits.
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Formula
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where:
- USL = Upper Specification Limit
- LSL = Lower Specification Limit
- σ (sigma) = Standard deviation of the process
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A higher Cp value means a more capable process, but it doesn’t consider if the process is centered.
Cpk (Process Capability Index):
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Measures the actual capability of a process, taking into account whether the process is centered or shifted.
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Formula:
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where μ (mu) is the process mean.
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If Cpk = Cp, the process is perfectly centered.
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If Cpk < Cp, the process is off-center.
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Interpretation of Cp and Cpk Values
- Cp, Cpk < 1.0 → Process is not capable (lots of defects).
- Cp, Cpk = 1.0 → Just meets specifications (barely capable).
- Cp, Cpk ≥ 1.33 → Process is capable (good quality).
- Cp, Cpk ≥ 1.67 → Highly capable process (six sigma quality aims for Cpk ≥ 2.0).