A PROCESS is a unique combination of tools, materials, methods, and people engaged in producing a measurable output; for example a manufacturing line for machine parts. All processes have inherent statistical variability which can be evaluated by statistical methods...
The Process Capability is a measurable property of a process to the specification, expressed as a process capability index (e.g., Cpk or Cpm) or as a process performance index (e.g., Ppk or Ppm). The output of this measurement is usually illustrated by a histogram and calculations that predict how many parts will be produced out of specification.
Process capability is also defined as the capability of a process to meet its purpose as managed by an organization's management and process definition structures ISO 15504.
Two parts of process capability are:
- Measure the variability of a process
- Compare that variability with a proposed specification or product tolerance
Measure The Process
The input of a process usually has at least one or more measurable characteristics that are used to specify outputs. These can be analyzed statistically, where the output data shows a normal distribution the process can be described by the process mean (average) and the standard deviation.
A process needs to be established with appropriate process controls in place. A control chart analysis is used to determine whether the process is "in statistical control". If the process is not in statistical control then capability has no meaning. Therefore the process capability involves only special cause variation and not common cause variation.
A batch of data needs to be obtained from the measured output of the process. The more data that is included the more precise the result, however an estimate can be achieved with as few as 17 data points. This should include the normal variety of production conditions, materials, and people in the process. With a manufactured product, it is common to include at least three different production runs, including start-ups.
The process mean (average) and standard deviation are calculated. With a normal distribution, the "tails" can extend well beyond plus and minus three standard deviations, but this interval should contain about 99.73% of production output. Therefore for a normal distribution of data the process capability is often described as the relationship between six standard deviations and the required specification.
Capability Study
The input of a process is expected to meet customer requirements, specifications, or product tolerances. Engineering can conduct a process capability study to determine the extent to which the process can meet these expectations.
The ability of a process to meet specifications can be expressed as a single number using a process capability index or it can be assessed using control charts. Either case requires running the process to obtain enough measurable output so that engineering is confident that the process is stable and so that the process mean and variability can be reliably estimated. Statistical process control defines techniques to properly differentiate between stable processes, processes that are drifting (experiencing a long-term change in the mean of the output), and processes that are growing more variable. Process capability indices are only meaningful for processes that are stable (in a state of statistical control).
For Information Technology, ISO 15504 specifies a process capability measurement framework for assessing process capability. This measurement framework consists of 5.5+0.5 levels of process capability from none (Capability Level 0) to optimizing processes (CL 5). The measurement framework has been generalized so that it can be applied to non IT processes.
There are currently two process reference models covering software and systems. The Capability Maturity Model in its latest version (CMMI continuous) also follows this approach.
Process Capability Index
In process improvement efforts, the process capability index or process capability ratio is a statistical measure of process capability: The ability of a process to produce output within specification limits.[1] The concept of process capability only holds meaning for processes that are in a state of statistical control.
If the upper and lower specifications of the process are USL and LSL, the target process mean is T, the estimated mean of the process is and the estimated variability of the process (expressed as a standard deviation) is , then commonly-accepted process capability indices include:
is estimated using the sample standard deviation.
Recommended Values
Process capability indices are constructed to express more desirable capability with increasingly higher values. Values near or below zero indicate processes operating off target ( far from T) or with high variation.
Fixing values for minimum "acceptable" process capability targets is a matter of personal opinion, and what consensus exists varies by industry, facility, and the process under consideration. For example, in the automotive industry, the AIAG sets forth guidelines in the Production Part Approval Process, 4th edition for recommended Cpk minimum values for critical-to-quality process characteristics. However, these criteria are debatable and several processes may not be evaluated for capability just because they have not properly been assessed.
Being the process capability a function of the specification, the Process Capability Index is as good as the specification is. For instance, if the specification came from an engineering guideline without considering the function and criticality of the part, a discussion around process capability is useless, and would have more benefits if focused on what are the real risks of having a part borderline out of specification. The loss function of Taguchi better illustrates this concept.
At least one academic expert recommends the following:
It should be noted though that where a process produces a characteristic with a capability index greater than 2.5, the unnecessary precision may be expensive.
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