Statistical Process Control Demystified by Paul Keller

Author:Paul Keller [Keller, Paul]
Language: eng
Format: mobi
Publisher: McGraw-Hill
Published: 2011-07-18T16:00:00+00:00

FIGURE 7.1 Example cause & effect diagram using Green Belt XL software.

Obviously, most internal or external supplies would not knowingly make changes that would certainly impact their customers. Similarly, process personnel would not engage in manipulating the process unless they felt the change insignificant (or on the contrary significant and beneficial) to the end result. It is thus paramount to engage internal and external suppliers and process personnel in a positive manner, without fear of retribution. Engaging these personnel is a critical first step toward building buy-in, which precedes any meaningful contribution toward process improvement.

In simple cases, the source of the special cause can be quickly identified and addressed: a procedure not followed; a worker untrained in a particular aspect of the process; inconsistent material from a supplier; a sudden change in environmental conditions. Other times, the source of the special cause variation requires further investigation, as is often the case when trying to identify significant sources of common cause variation.

Common cause variation, by its very nature, consists of the combined effect of many factors influencing the process. The combined effect on the process is what is common (as in common cause variation) to all affected subgroups, even though the factors that are significant in producing the effect may themselves vary in some predictable fashion. Understanding the relationship between a significant factor’s variation and the effect of that variation on the process is necessary to reduce common cause variation. This in itself is not a trivial analysis, yet is further complicated when a factor’s effect is influenced by the relative presence of other factors. Some occurrences of special cause variation are also manifested in this way: for example, if a supplier’s inconsistency is only a significant issue when the process is run at a higher throughput.

As a result, the process changes necessary to reduce these forms of special cause variation and almost all forms of common cause variation are themselves significant. As such, they demand sufficient management buy-in to allocate necessary resources and encourage cross-functional participation. A proper Six Sigma project, sponsored by management and executed by a cross-functional team of stakeholders led by a Black Belt trained in statistical problem solving, is perfectly suited to address these issues (Keller, 2011).

An overview of the techniques used to understand the sources of variation follows. A detailed discussion of the quantification of error attributable to measurement systems is found in Chapter 8.

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