1. 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

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

3. 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.

4. Interpreting the Process Capability Index
Cpk < 1 Not Capable
Cpk > 1 Capable at 3
Cpk > Capable at 4
Cpk > Capable at 5
Cpk > 2 Capable at 6

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

6. Process Capability Index, Cpk
Example problem solution:
= 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 } = (Not capable!)
However, by adjusting the mean, the process can become capable.