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Six Sigma and The Voice of the Process

- Dr Tony Burns

Six sigma is a specification based philosophy. It sets a defects target. This target is expressed as a function of the variability of the process. However, it is an expression of a requirement of the customer. There is much confusion between the voice of the process and the voice of the customer. A six sigma target of 4 defects (or product not conforming to specification) per million relates to the voice of the customer, not to the voice of the process.

The traditional tool for statistical process control is the 3 sigma control chart. Control charts are no different in six sigma programs than they were in earlier quality improvement programs. From their invention by Shewhart in 1924, control charts have stood the test of time.

The control chart monitors the voice of the process. The voice of the process is independent of where one chooses to set specification limits. Monitoring the voice of the process gives a measure of the predictability of the process. It is only when the process is in control that one can be sure that any level of defects, six sigma or otherwise, will be predictable. If points fall outside control limits, or if the mean changes, the process will not be predictable in its future behaviour. That is, defect may occur, no matter where specification limits are set.

Six sigma programs use exactly the same tools as traditional quality improvement programs, that is control charts, histograms, brainstorming and so forth. Six sigma programs have the same needs in training employees in the use and interpretation of the quality improvement tools. Despite extensive training of green and black belts, every employee need to be able to understand how and why improvement teams are working. Tools like Q-Skills can assist in this process:

There is also a need for tools that are easy to use by anyone. Q-Skills provides such tools, compatible with the training, such as its flowcharter with built-in logic checking.

In summary, six sigma relates to the voice of the customer and sets a specification limit. The voice of the process is the control chart. Tools such as Q-Skills can help reduce process variation.

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