Definitions
Process:
Process refers to any system of causes; any combination of conditions which work together to produce a given result.
Process Capability:
Process capability refers to the normal behavior of a process when operating in a state of statistical control. It refers to the inherent ability of a process to produce similar parts for a sustained period of time under a given set of conditions.
Process Capability Indices:
Process capability can be expressed as percent nonconforming or in terms of the natural spread related to the specification spread.
Process Capability Study:
A process capability study is a systematic procedure for determining the capability of a process. The process capability study may include studies to improve the process and in turn the capability of the process. Process capability studies are usually performed as part of a process certification effort or a process optimization effort.
Process Capability Study Steps
The general process capability study steps are:
1. Select Critical Parameters
Critical parameters need to be selected before the study begins. Critical parameters may be established from drawings, contracts, inspection instructions, work instructions, etc. Critical parameters are usually correlated to product fit and/or function.
2. Collect Data
A data collection system needs to be established to assure that the appropriate data is collected. It is preferable to collect at least 60 data values for each critical parameter. If this is not possible, corrections can be made to adjust for the error that is introduced when less than 60 data values are collected. Significant digits for each data should be the number of significant digits required per the specification limits plus one extra significant digit to assure that process stability can be evaluated.
3. Establish Control over the Process
A distinction between product and process should be made at this point. The product is the end result from the process. The product may be a physical item (Example: fabricated part) or a service (example: typing a report). One may control the process by measuring and controlling parameters of the product directly or measuring and controlling the inputs to the process (once correlation between the process inputs and product critical parameters have been established). It is ultimately desirable to establish control over the process by controlling the process inputs. On the other hand, process capability indices are always performed using the critical parameters of the product. Calculation of predictable process capability indices is dependent on the statistical control of the process. If the process is not in statistical control, then the results of the study are subject to fluctuate unpredictably. The statistical control of the process can be studied using control charts (usually Xbar-R charts).
4. Analyze Process Data
To calculate the process capability indices, estimates of the process average and dispersion (standard deviation) must be obtained from the process data. In addition, the formulas for process capability indices assume that the process data came from a normal statistical distribution. It is important that one prove that the data is normally distributed prior to reporting the process capability indices because errors in misjudgment can lead to the same undesirable effects as listed in Step 1. Methods for handling non-normal data and formulas for several process capability indices will be addressed in separate sections.
5. Analyze Sources of Variation
Study of the component sources of variation and their magnitudes may range from simple statistical tests to complex experimental designs carried out over a long period of time. If possible, tests should be kept simple. Analyzing sources of variation involves determining what process factors affect the natural process spread (process variation) and the process centering. With this knowledge, it may be possible to improve the process’ capability. Analyzing sources of variation always involve careful planning and data collection.
6. Establish Process Monitoring System
Once the process capability indices indicate a capable process, a routine process control technique should be employed to assure that the process remains stable. This may be done by a variety of methods such as establishing a statistical process (SPC) program. The process capability indices should also be periodically recalculated to assure the process mean and spread has not significantly changed.
