Process Capability Studies

Process Capability Studies - Process Capability Studies are short-term studies conducted to obtain early information on the performance of new or revised processes relative to internal or customer requirements. In many cases, preliminary studies should be conducted at several points in the evolution of new processes (e.g., at the equipment or tooling subcontractor's plant, after installation at the supplier's plant) These studies should be based on as many measurements as possible. When X-Bar (Xbar) and R charts, at least twenty subgroups (typically three to five pieces, when taking sub-groups) are required to obtain sufficient data for decision making. When this amount of data is not available, control charts should be started with whatever data is available. >>

   Formulae:
   Cp=(USL-LSL)/6s
   Cpu=(USL-Xbar)/3s
   Cpl=(Xbar-LSL)/3s
   Cpk=Minimum of (Cpu,Cpl)

Process Capability, Cpk uses 's' or the population standard deviation which is estimated using (Rbar/d2) or (Sbar/C2). Potential Process Capability, Ppk uses the 's' or the sample standard deviation from individual data. Ppk attempts to answer the question "does my 'sample' meet specification?" Cpk attempts to answer the question "does my process meet specification?" A Cp value significantly greater than the corresponding Cpk indicates an opportunity for improvement by centering the process.

Note: Per AIAG handbook - "the process must first be brought into statistical control by detecting and acting upon special causes of variation. then its performance is predictable, and its capability to meet customer expectations can be assessed. this is the basis of continual improvement." Do not be tempted to rearrange the data in order to get a higher Cpk. data should always be gathered in an appropriate manner (eg: appropriate subgroup size) and ordered by time.

[Category=Quality ]

Source: The Quality Portal, 18 April 2011 09:22:12, http://thequalityportal.com/glossary/l.htm