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Process Capability (Cp / Cpk)

Cp = (USL−LSL)/6σ measures spread; Cpk = min[(USL−μ), (μ−LSL)]/3σ also penalises being off-centre. Cp = Cpk only when perfectly centred. PPM assumes a normal distribution.

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Mean μ

Std deviation σ

Upper spec limit

Lower spec limit

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CPU

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CPL

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PPM above USL

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PPM below LSL

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Total PPM

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Assumes a stable, normally distributed process. A helper, not a substitute for engineering judgment.