Process Capability Process limits (The “Voice of the Process” or The “Voice of the Data”) - based on natural (common cause) variation Tolerance limits (The “Voice of the Customer”) – customer requirements Process Capability – A measure of how “capable” the process is to meet customer requirements; compares process limits to tolerance limits

Process Capability common variation specification (1)(1) common variation (3)(3) specification common variation (2)(2) specification common variation (4)(4)

Process Capability Index where x-bar is the mean of the process, sigma is the standard deviation of the process, UTL is the customer’s upper tolerance limit (specification), and LTL is the customer’s lower tolerance limit.,

Interpreting the Process Capability Index C pk < 1Not Capable C pk > 1Capable at 3  C pk > 1.33Capable at 4  C pk > 1.67Capable at 5  C pk > 2Capable at 6 

Process Capability Index, C pk A process has a mean of 45.5 and a standard deviation of 0.9. The product has a specification of 45.0 ± 3.0. Find the C pk.

Process Capability Index, C pk = min { (45.5 – 42.0)/3(0.9) or ( )/3(0.9) } = min { (3.5/2.7) or (2.5/2.7) } = min { 1.30 or 0.93 } = 0.93 (Not capable!) However, by adjusting the mean, the process can become capable. Example problem solution: