Process Capability study is a way to measure the performance and capability of a process to meet customer specifications. The Process Capability Index (Cpk) uses both the process variability and the process specifications to determine whether the process is “capable”. The larger the index, the less likely it is that any item will be outside the customer specifications.
Example
Steel cylinders manufactured by a company should have their diameters-as specified by customers- between 42 and 54 mm. The current process produces cylinders with average diameter of 49 mm with a 2-mm standard deviation. The capability of this process to meet customer specifications can be measured with the Cpk as the minimum between two values as represented in the formula:
where,
USL: the customer upper specification limit (54)
LSL: the customer lower specification limit (42)
mu: the mean diameter produced by the process (49)
Sigma: the process standard deviation (2)
About the Calculator
This application helps you calculate your process capability index by providing the above as input variables. Additionally, it calculates the Defect Per Million Opportunity (DPMO) which is the expected total number of defects per million defect opportunities produced. This is represented by the red area under the curve outside the lines of specification limits.
Besides, the application calculates the Sigma Level of the process using the formula:
Sigma Level is a measure of the defect rate of a process based on the DPMO estimate. The higher the Sigma Level, the better the performance is. For example, a process of 6-sigma level has 3.4 defects per million opportunity. Whereas a process of 3-sigma level has 66,807 DPMO.