Cp/Cpk Test
The Cp/Cpk test assesses the capability of a process in terms of repeatability and accuracy, in order to verify whether the process meets specified requirements.
This test analyzes the process result values in relation to a tolerance range defined by an upper limit and a lower limit.
The test assessment is based on two indexes:
The Cp index describes the process capability meant as the number of times the spread of the result values fits into the tolerance range. This means the Cp index describes how close the collected result values are to one another, determining the repeatability of the process without taking into account how these values are positioned in relation to the upper and lower tolerance limits.
The Cpk index describes the process capability corrected by the position of the result values within the tolerance range. This means the Cpk index expresses how close the collected result values are to the target value (the middle of the tolerance range) determining the accuracy of the process in addition to its repeatability.
A | Upper tolerance limit | D | Cp: Low / Cpk: Low |
B | Target value | E | Cp: High / Cpk: High |
C | Lower tolerance limit | F | Cp: High / Cpk: Low |
If Cpk > 1 and Cp > 1, the process is capable and the process results are within the tolerance limits.
If Cpk < 1 and Cp < 1, the process is not potentially capable and the process results are not within the tolerance limits.
If Cpk < 1 and Cp > 1, the process is potentially capable, but it is not centered and the results are non within the tolerance limits.
