WHAT IS PROCESS
CAPABILITY?
Quality Glossary Definition: Process capability
Process capability is defined as a statistical measure of the inherent process variability of a given
characteristic. You can use a process-capability study to assess the ability of a process to meet
specifications.
During a quality improvement initiative, such as Six Sigma, a capability estimate is typically obtained
at the start and end of the study to reflect the level of improvement that occurred.
Several capability estimates are in widespread use, including:
• Potential capability (Cp) and actual capability during production (Cpk) are process
capability estimates. Cp and Cpk show how capable a process is of meeting its specification
limits, used with continuous data. They are valuable tools for evaluating initial and ongoing
capability of parts and processes.
• "Sigma" is a capability estimate typically used with attribute data (i.e., with defect rates).
Capability estimates like these essentially reflect the nonconformance rate of a process by
expressing this performance in the form of a single number. Typically this involves calculating some
ratio of the specification limits to process spread.
ASSESSING PROCESS CAPABILITY
Assessing process capability is not easy. Some textbooks teach users to wait until the process
reaches equilibrium, take roughly 30 samples and calculate their standard deviation; however, it is
difficult to know when the process reaches a state of equilibrium and if the recommended samples
are representative of the process. The measurement of process capability is more complicated than
that.
For example, suppose you have a rotary tablet press that produces 30 tablets, one from each of 30
pockets per rotation. If you’re interested in tablet thickness, you might want to base your estimate of
process capability on the standard deviation calculated from 30 consecutive tablets. Better yet, you
might assure representation by taking those 30 consecutive tablets repeatedly over eight time
periods spaced evenly throughout a production run (Table 1). You would pool the eight individual
standard deviations yielding a thickness capability estimate based on (8 X (30 - 1)) = 232 degrees of
freedom.
Table 1: Assessing Process Capability: Sampling Scheme Example
For greater assurance yet, you might want to include several production runs with perhaps fewer
sampling times per production run. Estimates of the process capability made this way would be
representative and independent of process mean changes that might take place from one sampling
time to the next. Because the pooled, within-group standard deviation is calculated on observations
taken close together in time, there is no opportunity for it to be contaminated by assignable sources
of variation. It is as close to pure capability as you’re likely to get.
PRACTICAL CONCERNS WHEN
CONDUCTING CAPABILITY
STUDIES
There are both positive and negative aspects to capability estimates. For example, Cp and
Cpk estimates are highly sensitive to the assumption that one is sampling from a normal distribution—
that is, most of the data points are concentrated around the average (mean), forming a bell–shaped
curve.
Furthermore, sampling from a stable system is essential to obtaining meaningful estimates of
process performance for future production.
Many quality practitioners report solely the numerical values of the capability estimates. Others,
however, note that the capability estimates are themselves merely statistics, or point estimates of
the true capability of a process. As such, the use of confidence intervals for the true capability values
may also be reported.
When sampling from stable, but non-normal distributions, other strategies to obtain meaningful
capability estimates may be appropriate, including:
• Transforming the data to be approximately well modeled by a Normal distribution.
• Using an alternative probability distribution, such as Weibull or lognormal distributions.
PROCESS CAPABILITY
RESOURCES
You can also search articles, case studies, and publications for process capability resources.
Modern Approach (Quality Progress) Traditional process capability analysis no longer is the best
way to model performance in today’s digital age, where dynamic environments and remote process
monitoring require more rapid data analysis cycles to support automation. The authors propose a
new method that applies process capability and stability concurrently, which allows samples to be
refreshed more frequently, thereby capturing the dynamic shifts in processes.
My Supplier's Capability Is What? (Quality Progress) Know what processes your suppliers are
using, and make them prove those processes are capable and controlled.
Process Evaluation: Capability Analysis Of Complex Parts (PDF) The background and
implications of the Military Aircraft and Missile Systems Group developing appropriate methods for
monitoring processes and evaluating process capability.
Assessing Process Capability section adapted from "What's Meant by 'Capability'?" (Quality
Progress).