What Are the Formulas for CP and Cpk in Process Capability Analysis?

Learn the key formulas for CP and Cpk, including definitions of USL, LSL, mean, and standard deviation in process capability.

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CP (Process Capability) is calculated using the formula: CP = (USL - LSL) / 6σ. Cpk (Process Capability Index) is calculated as: Cpk = min( (USL - μ) / 3σ , (μ - LSL) / 3σ ). USL is the Upper Specification Limit, LSL is the Lower Specification Limit, μ is the process mean, and σ is the standard deviation.

FAQs & Answers

  1. What is the difference between CP and Cpk? CP measures the potential capability of a process assuming it is centered between specification limits, while Cpk accounts for how centered the process is relative to those limits, providing a more realistic capability measure.
  2. What do USL and LSL stand for in process capability calculations? USL stands for Upper Specification Limit and LSL stands for Lower Specification Limit; these define the acceptable range for a process output.
  3. How are the mean and standard deviation used in CP and Cpk? The process mean (μ) represents the average value of the process, and the standard deviation (σ) measures variation. CP uses these to calculate spread, while Cpk also considers how closely the mean is centered.

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