Understanding Process Capability Indices

Understanding Process Capability Indices


1.

Introduction
A process capability index is a numerical summary that compares the behavior of a product

or process characteristic to engineering specifications. These measures are also often called
capability or performance indices or ratios; we use capability index as the generic term. A
capability index relates the voice of the customer(specification limits) to the voice of the process.
A large value of the index indicates that the current process is capable of producing parts that, in all
likelihood, will meet or exceed the customer’s requirements. A capability index is convenient
because it reduces complex information about the process to a single number. Capability indices
have several applications, though the use of theindices is driven mostly by monitoring
requirements specified by customers. Many customers ask their suppliers to record capability
indices for all special product characteristics on a regular basis.

The indices are used to

communicate how well the process has performed. For stable or predictable processes, it is
assumed that these indices also indicate expected future performance.Suppliers may also use
capability indices for different characteristics to establish priorities for improvement activities.
Similarly, the effect of a process change can be assessed by comparing capability indices calculated
before and after the change.
Despite the widespread use of capability indices in industry, and some good review
articles, such as Gunter (1989abcd), there is much confusion andmisunderstanding regarding their
interpretation and appropriate use. This problem is magnified because many quality programs,
such as, for example, the automotive industry standard QS-9000 mandate the use of capability
indices, but state the capability requirements in an overly complex and confusing way.

The

following three excerpts from the QS-9000 reference manuals illustrate theproblem. The process
capability requirements given in Section 4.9.3. of the manual “Quality Systems Requirements”
(Automotive Industry Action Group (AIAG), 1995) state, in part:
“Ongoing process performance requirements are defined by the customer. If no
such requirements have been established, the following default values apply:
• For stable processes and normally distributed data, a Cpk value ≥1.33 should
be achieved.
• For chronically unstable processes with output meeting specification and a
predictable pattern, a Ppk value ≥ 1.67 should be achieved.”
In the Advanced Product Quality Planning (APQP) manual (AIAG, 1995), the Ford Powertrain
specific requirements for dynamic control plans (DCP, Appendix G) say, in part:
“All processes must produce all characteristics tospecification on a production
basis. ... Significant Characteristics (SCs) must be in a state of statistical control
with Ppk ≥ 1.67 and Cpk ≥ 1.33.”
Finally, within the Production Part Approval Process (PPAP) manual (AIAG, 1995), the
requirements that relate to process capability are given as follows:
“Calculate the Ppk index and take the following actions:
For Processes that Appear Stable
ResultsInterpretation
Pp and Ppk > 1.67 The process probably meets customer requirements. After
approval, begin production and follow the Approved
Control plan.
1.33 ≤ Ppk ≤ 1.67 The process may not meet customer requirements. After
part approval, begin production with additional attention to
the characteristic until an ongoing Cpk ≥ 1.33 is achieved.
Ppk < 1.33

The process is substandard formeeting customer
requirements. Process improvements must be given high
priority and documented in a corrective action plan.
Increased inspection or testing is normally required until an
ongoing Cpk of 1.33 is demonstrated. A revised control
plan for these interim actions must be reviewed with and
approved by the customer.

[Processes that appear unstable at the time PPAP approval is sought...