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Understanding Process Variation

Autor : Dede Ericson

The definition of a process is a series of actions or steps taken to achieve a particular end. Once a process is established, the desire is to have these actions or steps followed to ensure a repeatable outcome occurs regardless of who or when the actions are taken. If we do not know how something was done or how we got here, then we will not be able to achieve the same results. This sort of consistency is key in business processes with products and customers!


What is Process Variation?

Process variation can be defined as the numerical value used to indicate how widely individuals in a group vary. Process variation happens when production processes fail to follow a precise pattern. This is a leading cause of quality issues in production and transactional processes. Many times, product quality issues are not identified until it has turned into a large-scale disaster. Now, leadership must identify the source of the problem and take the needed steps to prevent the problem from reoccurring. If there is too much process variation, identifying areas for improvement becomes challenging.

Some level of variance in manufacturing processes is inevitable. The primary question is whether the inputs that cause the variation are controlled and predicted. If you understand this variation, can account for it in the output, or work around it, then we can handle the situation. However, having a general knowledge of process variation and the types can provide a solid foundation for improvement.

Types of Process variation

Generally, there are two types of process variations:

  1. common cause
  2. special cause

process variation graph


Common Cause Variation

Common cause variation, also referred to as “noise” results from things that may or may not be known. The causes for this variance are usually quantifiable and natural in the system. Common cause variation usually lies within three standard deviations from the mean where 99.73% of values are expected to be found. If you are plotting on a control chart this variation is indicated by a few random points that are within the control limit. The impact common cause variation has on the process outputs is controllable and predictable resulting in a statistically stable process. To eliminate this common cause variation, you will have to make a fundamental change.


Special Cause Variation

Special cause variation refers to unexpected things that affect a process. These variations were not observed, are unusual and non-quantifiable. These sporadic causes are the result of a change introduced in the process resulting in a problem. Things can get more challenging when special cause variation is introduced because they occur out of the blue. This failure can be corrected by making changes in the methods, material, or processes.


Process Variation in Manufacturing

An example of common cause variation in a manufacturing process is an oven or finishing range with a thermostat that allows the temperature to slightly drift up and down but remain within the control limits.

Special cause variation in a manufacturing environment may include environment, materials, manpower, technology, equipment, and many more. Utilizing the same manufacturing example as earlier of an oven or finishing range with a thermostat, if that thermostat “fails” resulting in a rapid spike or drop in temperature, this would result in the manufacturing process being out of control and a special cause variation. Another example of special cause variation could be when a new employee is hired and placed directly on the job with no training. There is no knowledge of the process resulting in end of the day output data outside the control limits.


The Benefit of Understanding Process Variation

Process variation can help direct where to work and drive continuous improvement. Identifying, characterizing, quantifying, and reducing variation can enhance financial performance, reduce operating costs, and improve customer satisfaction. We should be continually monitoring for sources of variation and aggressively working to reduce variation with the knowledge that this will improve overall performance.

Performance Solutions by Milliken

For companies in need of immediate recovery or transformation, our Rapid Improvement (RI) process pinpoints specific principles of the Performance Management System and escalates implementation with focused precision providing a speedy approach to specific needs. Some examples include:

  • Working on the method element of the lean Six Sigma elements that contribute to process variation – Method, Mother Nature “Environmental”, (Man) People, Measurement, Machine, Materials – through the creation of Standard Operating Procedures and training employees how to consistently complete a process
  • Implementing a Daily Management System to ensure the leader standard work includes layered audits to monitor and sustain gains achieved through standardize operating procedures

If you are ready to focus your improvement efforts to generate wins through eliminating special cause variation and focus on common cause variation to create more consistent, stable process, Performance Solutions by Milliken can help. Contact us today.