Defects in Six Sigma Methodology

Imagine a process whose performance is represented by the graph below. The line in the center represents both the average of the process measure and the target performance level. The lines to the left and right are the lower and upper specification limits. In other words, to meet the customer definition of quality or the physical product requirements, the measure must fall between the lower and upper spec limits.

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Defects play a central role in the Six Sigma methodology. The very name “Six Sigma” represents its focus on reducing variation and establishing performance so that fewer than four defects occur in a million opportunities. This occurs when the process variation or sigma level is 6.0. While this goal is more appropriate for manufacturing than service organizations, the name stuck.

In most cases, a defect should be thought of as a failure to meet the customer expectations for quality. While it is tempting to assume that business leaders know what quality is for a specific product or process, a central tenet of the Six Sigma methodology is that the customer defines quality. In determining customer specifications, companies may rely on surveys, focus groups, customer complaints, returns and other sources of information. Once specifications are correctly established, it should be fairly simple to determine what a defect is relative to those specs.

It is also important to have a clear understanding of who the customers are for a specific product or process. In some cases, there are internal customers – other departments, for example – rather than external customers. In addition, it is sometimes appropriate to establish market segments for different types of customers, and determine quality expectations separately for each segment.

The goal of many Six Sigma DMAIC projects is to improve performance and satisfaction by reducing the defect rate. Once the defect(s) for a process have been defined via the Voice of the Customer process in the Define phase, the current defect rate can be measured in the Measure phase. Then in the remainder of the DMAIC process, the team determines root causes for the defects, implements improvements to counter those root causes, and establishes a means of ensuring the gains are maintained. For more information about the DMAIC methodology, read my article about DMAIC and my 5-part series on the 5 phases.