Using Six Sigma to
Improve Product Quality
How can Six Sigma improve product quality? The key is to reduce variation and eliminate defects. Let’s explore the basics of Six Sigma to understand how this is accomplished.
The term "sigma" refers to standard deviation, which is a measure of the variation or scatter in a process. Within business and industry, the sigma value is a metric that indicates how well a process is performing, compared to the benchmark value of Six Sigma. Sigma measures the capability of a process to perform defect-free work. A defect is anything that may result in customer dissatisfaction.
The common measurement for Six Sigma is defects-per-unit, where a unit can be virtually anything: a component, an administrative form, a piece of material, a line of software code, and so on. The sigma value is a quality measurement that indicates how often a defect is likely to occur. The higher the sigma value, the less likely a process will produce defects. As sigma increases, cycle time and cost decreases, and customer satisfaction increases.
So what does it mean to be Six Sigma? Consider a process that produces one million parts. For this process to meet a Six Sigma quality level, it must produce less than four defective parts (the actual number is 3.4) out of the million that are produced! Clearly, achieving a Six Sigma quality level represents world-class status.
Let’s further examine the impact of variation on product quality. Referring to the graphic, the variation of two products is depicted and represented by the bell-shaped curves. The product produced using Six Sigma methodologies is shown with less variation, represented by the steeper slope of the curve and more narrow spread around the mean value, than the traditional product.
With the customer specification limit superimposed on the curves, you can see that the shaded area under the curves on the right-hand side of the specification limit line is considerably smaller for the Six Sigma product than for the traditional product. This area corresponds to the quantity of defects that are produced for each product.
Clearly, a Six Sigma product produces far fewer defects translating into less scrap and rework costs. As a result of this reduced variation, Six Sigma methodologies often lead to the identification of product development best practices. Ultimately, exploiting these practices result in the creation of superior products.
As you can see, variation has a significant impact on product quality. Controlling variation leads to improved productivity and lower costs, which translate into a competitive advantage for the company.
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