Introduction to Industrial Engineering

By Jane M. Fraser

Chapter 5

The IE Approach


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5.5 Six Sigma

Consider the following three targets, showing where arrows hit a target when shot by three different archers.

Target 1

In Target 1, all the holes are in the center, showing that the first archer has consistently put the arrows where they should be.

Target 2

Target 2 shows a tight cluster of arrow holes, but that cluster is not in the center. The second archer is being consistent, but the archerís aim is off.

Target 3

On Target 3, the holes are scattered around the center, but with a large amount of dispersion. The third archer must be using different methods each time.

Target 1 represents desired performance. How can we get the second and third archers to achieve that desired performance? The second archer needs only to readjust the aim and the tight cluster of arrow holes will be in the desired location. However, what can we tell the third archer to do? The third archer needs to focus first on being consistent. That archer must be shooting the arrow differently each time. Achieving consistency will be hard work requiring looking at every part of the process the archer is using.

Six Sigma emphasizes using data and quantitative analysis to reduce variability. If the variability in a process can be reduced, then the process can be centered to produce items with the required specifications. [Show 3 Normal densities; first is narrow and centered; second is narrow but not centered; third is centered but wide.]

Six Sigma aims to achieve a six sigma performance target (only 3.4 defects out of a million opportunities) for product and service characteristics that are critical to quality, that is, the characteristics that matter most to customers (Harry and Schroeder, page 13).

A six sigma level of defects is a dramatic improvement over the defect rate of the average company, which is 3.5 to 4 sigma. Harry and Schroeder point out that 1500 square feet of wall-to-wall carpet cleaned to a three sigma level would still have about four square feet of uncleaned carpet, while a six sigma level would mean the uncleaned area would be the size of a pinhead (Harry and Schroeder, pages 14-15). The resulting quality saves money for the company by reducing the need for rework. As a customer, you are likely to call back the three sigma carpet cleaners and make them redo the work correctly (at their cost), but you wonít call back the six sigma cleaners. The six sigma organization has dramatically reduced the cost for rework.

Production processes need to be consistent in efficiency, quality and safety. Six Sigma seeks to reduce the variability in the time and resources used to perform a task, the measurements concerning a product or service that indicate value for the customer, and the processes that ensure safety. Every task is done right the first time. Because defects are reduced, costs are reduced: costs for rework, repair, handling customer complaints, and warranties. In fact, Phil Crosby argues that quality is free because increased quality decreases costs so much.

How does Six Sigma achieve this increase in consistency? The DMAIC steps that you have already read about came from Six Sigma. Six Sigma really is another version of industrial engineering, involving the same steps to achieve continuous improvement.

Six Sigma uses the following tools, in addition to the tools listed earlier: