It provides insight into how close the process is to 6 sigma, but it does not give guidance on what should change in the process.FormulaThe formula for Cp is (UCL – LCL) / (6 x sigma) which subtracts the lower control limit (LCL) from the upper control limit (UCL) and divides the result by the product of 6 times sigma.
InterpretationThe closer the Cp is to 2, the closer the process is to achieving 6 sigma.
When To UseUse process capability at the problem identification (or definition) stage to measure the required improvements and to implement them.
Cp vs. Cpk is calculated differently and is the smaller of ((UCL – Mean) / 3(standard deviation)) and ((Mean – LCL) / 3(standard deviation)).
CommunicationSpeak in terms of the number of defects eliminated instead of process capability. However, do not use Cp if the mean of the process is not centered between the UCL and the LCL. Use everyday language to describe improvements.
Source:Quality Toolbook (Syque.com): Process Capability: When To Use It
NIST.gov: What Is Process Capability?
ISixSigma.com: Communicating Process Capability
More Information:NIST/SEMATECH e-Handbook of Statistical Methods
American Society for Quality (ASQ.org): Six Sigma Forum
. Cp (process capability) measures the capability of a process to meet the customers’ needs. CpkCp and Cpk (Process Capability Index) are process capability indexes. In other words, state that the improvement eliminated 19,283 defects instead of stating the process moved from 3 sigma to 4 sigma. When a process is Six Sigma, it means that there are only 3.4 defects per million opportunities
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