Capability ratios (Cp/Pp) describe the variability of a process relative to the
A capability ratio is a unit-less value describing the ratio of process distribution spread to specification limits spread. 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). The higher the value, the more capable the process of meeting specifications. A
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.
All of the indices assume a normally distributed process quality characteristic with the parameters specified by the process mean and sigma. The process sigma is either the short-term or long-term sigma estimate.
Indices computed using the short-term sigma estimate are called Cp indices (Cp, Cpl, Cpu, Cpk, Cpm). While those using long-term sigma estimate are called Pp indices (Pp, Ppl, Ppu, Ppk, Ppm). 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.
We've been developing intuitive high-quality statistical software at an affordable price, backed up by fast friendly customer service for over 25 years...
Office hours: 9am-5pm GMT Mon-Fri
It's Tuesday 5:02 PM and the office is closed - e-mail us and we'll get back to you as soon as possible.