What is it: A manufacturing process is a unique combination of tools, materials, methods, and people engaged in producing a measurable output, and because of these inputs there will be an inherent statistical variability in a process when producing products. The Process Capability studies this inherent variability to ensure statistical that the process is in control and that no adverse conditions is present. The process Capability Study will compare the process specification to the process output and determine statistically if the process can meet the customer’s specification.
Process Capability is the total range of inherent variation in a stable process. It is determined using data from control charts. The control charts shall indicate stability before capability calculations can be made. Histograms are to be used to examine the distribution pattern of individual values and verify a normal distribution. When analysis indicates a stable process and a normal distribution, the indices Cp and Cpk can be calculated. If analysis indicates a non-normal distribution, advanced statistical tools such as PPM analysis, will be required to determine capability. If control charts show the process to be non-stable, the index Ppk can be calculated, as the Ppk takes consideration of the centering of the process. The main indicators used for process capability are:
Why use it: The less variation there is in a process, the more capable it will be of meeting the customer’s specification.
Where to use it: When setting up a new process or modifications to a new process.
When to use it: To determine if the manufacturing process is stable and not changing, and can fit within the customer’s specifications.
How to use it: The primary measures of Process Capability used are the Process Capability ratio, the Process Capability index, and the Cpk.
The output 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 common cause variation and not special 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.
Important Notes: When using Process Capability as a means for measuring how well our process is producing compared to the customer’s requirements, need to make sure that the process is capable; but also it must also be fairly well-centered.