If the Cp indices are much smaller than the Pp indices, it indicates that there are improvements you could make by eliminating shifts and drifts in the process mean. While those using long-term sigma estimate are called Pp indices (Pp, Ppl, Ppu, Ppk, Ppm). Indices computed using the short-term sigma estimate are called Cp indices (Cp, Cpl, Cpu, Cpk, Cpm). The process sigma is either the short-term or long-term sigma estimate. Value of 2 or higher is required to achieve Six Sigma capability which is defined as the process mean not closer than six standard deviations from the nearest specification limit.Īll of the indices assume a normally distributed process quality characteristic with the parameters specified by the process mean and sigma. 13.0 Process Capability Special Topics Purpose Objectives Short-term versus Long-term Capability Studies Cp vs. The higher the value, the more capable the process of meeting specifications. A value of less than 1 is unacceptable, with values greater than 1.33 (1.25 for one-sided specification limits) widely accepted as the minimum acceptable value, and values greater than 1.50 (1.45 for one-sided specification limits) for critical parameters (Montgomery, 2012). ![]() Capability ratios (Cp/Pp) describe the variability of a process relative to theĪ capability ratio is a unit-less value describing the ratio of process distribution spread to specification limits spread.
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