What is CP and Cpki in SPC?

Statistical Process Control (SPC) is a quality control method used to monitor and control processes to ensure they are operating within specified limits. It involves collecting and analyzing data to identify trends, patterns, and variations in the process.

Understanding CP

CP, also known as Process Capability, is a measure that quantifies the ability of a process to produce output that meets customer specifications. It provides an indication of how well a process can perform in relation to its upper and lower specification limits. CP is calculated using the formula:

CP = (USL - LSL) / 6σ

Where USL represents the upper specification limit, LSL represents the lower specification limit, and σ represents the standard deviation of the process.

Interpreting CP Values

The resulting CP value helps determine if a process is capable of producing output within the specified tolerances. Generally, a CP value greater than 1 indicates that the process is capable of meeting the specifications. The higher the CP value, the better the process's capability to consistently produce output within the tolerances.

However, it's important to note that CP only considers the spread of the process relative to the specification limits and doesn't take into account the centering of the process. To evaluate both aspects, another metric called CPK is used.

Understanding CPK

CPK, also known as Process Capability Index, measures the capability of a process while considering both its spread and centering. CPK takes into account the distance between the process mean and the nearest specification limit, as well as the spread of the process.

CPK is calculated using the formula:

CPK = min((USL - μ) / 3σ, (μ - LSL) / 3σ)

Where μ represents the process mean. CPK values range from 0 to 1, where a CPK of 1 indicates that the process perfectly meets the specifications. A CPK value less than 1 indicates that the process has potential for non-conformance.