What You Should Know About Process Capability
Introduction
A process can be defined as a sequence of procedures, operations or steps that convert inputs into outputs. Every process has certain amount of variation which can be measured, monitored, reduced and/or controlled. By taking accurate measurements using reliable, well-maintained equipment, variation in a process can have little impact on the quality of the output hence able to produce acceptable product on a consistent basis.
Process Capability Defined
Process capability is defined as a statistical measure of the process variability of a given characteristic which can be used to assess the ability of a process to meet specifications. To determine how a process is operating, we can calculate:
- Process Capability (Cp)
- Process Capability Index (Cpk)
- Preliminary Process Capability (Pp)
- Preliminary Process Capability Index (Ppk),
The Cp and Cpk indices are used in evaluating existing, an already established process in statistical control. While the Pp and Ppk indices are used in evaluating a new process or one that is not in statistical control. Cp determines if a process is capable of making parts within specifications and Cpk determines if the process is centralized between the specification limits.
The Cp index is a fundamental indication of process capability which is calculated using the specification limits and the standard deviation of the process. Most organizations require that the process Cp = 1.33 or greater.
The Cpk index on the other hand, examines how close a process is performing to the specification limits putting into consideration the common process variation. The larger the value of the Cpk, the closer the mean of the data is to the target value. Cpk can be calculated using the specification limits, standard deviation or sigma, and the mean value. The value should be between 1 and 3. If the value is less than 1, the process is in need of improvement.
Accurate process capability study depends upon four basic assumptions regarding the data:
- No special causes of variation in the process.
- Process is in a state of statistical control.
- The data used for study fits a Normal distribution, exhibiting a bell shaped curve and can be calculated to +/- 3 sigma.
- The sample data is true representative of the population randomly collected from a large production run.
Why Measure Process Capability
Monitoring process capability allows the manufacturing process performance to be evaluated and adjusted as needed in order to assure that products meet the design or customer’s requirements. When used effectively, the information derived from the study can help reduce waste, improve product quality and consistency, lower cost of manufacturing and the cost of poor quality.
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About the Author
Adebayo is a thought leader in continuous process improvement and manufacturing excellence. He is a Certified Six Sigma Master Black Belt (CSSMBB) Professional and Management Systems Lead Auditor (ISO 9001, 45001, ISO 22000/FSSC 22000 etc.) with strong experience leading various continuous improvement initiative in top manufacturing organizations.
You can reach him here.
