How six sigma philosophy is aligned with process quality improvement?

Module II. Process Quality Improvement

Lecture-9 How six sigma philosophy is aligned with process quality improvement?

High-technology products with many complex components typically have many opportunities for failure or defects to occur. Motorola developed the six-sigma program in the late 1980s as a response to the demand for these products. The focus of six-sigma is reducing variability in key product quality characteristics, or so-called CTQ, to the level at which defects are extremely unlikely.

Figure 2-29 shows a normal probability distribution as a model for a quality characteristic with the specification limits at six standard deviations on either side of the mean.

Now it turns out that in situation when specification lines are at three standard deviation level, the probability of producing a product within these specifications is 0.9973, which corresponds to 2700 parts per million (ppm) defective. This is referred to as three-sigma quality performance. In case we have a product that consists of an assembly of 100 components or parts and all 100 of these parts must be no defects for the product to function satisfactorily. The probability that the unit of product is having no defects is

0.9973 x 0.9973 x ... x 0.9973 = (0.9973)¹⁰⁰= 0.7631

That is, about 23.7% of the products produced under three-sigma quality will be defective. This may not an acceptable situation, because many high-technology products are made up of thousands of components. An automobile has about 200,000 components and an airplane has several million.

The Motorola six-sigma concept is to reduce the variability in the process so that specification limits are six standard deviations from the mean. Then, a s given in Figure 2-30, there will only be about 2 parts per million defects. For six-sigma quality, the probability that any specific unit of the hypothetical product is having defect is 0.9999998, or 0.2 defect parts per million.

When the six-sigma concept was initially developed, an assumption was made that when the process reached the six-sigma quality level, the process mean was still subject to disturbances that could cause it to shift by as much as 1.5 standard deviations off target. Under this scenario (shown in Figure 2-30), a six-sigma process would produce about 3.4 ppm defects.

Six sigma also considers an important concept of opportunity to define the PPM level. Motorola established six-sigma as both an objective for the corporation and as a focal point for process and product quality improvement efforts. In recent years, six-sigma has spread beyond Motorola and has come to encompass much more. It has become a program for improving corporate business performance by both improving quality and paying attention to reducing costs. Companies involved in a six-sigma effort utilize teams to work on projects that have both quality and significant financial impact. The effort is better focused than in earlier TQM programs, and has been more successful in obtaining management commitment. However, remember Deming's point 10, which essentially says to eliminate slogans and programs to improve quality. There are many programs including zero defects, value engineering, quality is free, TQM, and so forth, which has failed due to improper implementation. A major component in successful quality improvement is driving the use of the proper statistical and engineering tools into the right places in the organization. A DMAIC (Define, Measure, Analyze, Improve and Control) approach is used to implement six sigma philosophy. However, it is to be remembered that statistical approach (say SPC, DOE) is the key theme for variation reduction in Six Sigma philosophy. Some additional information on Six Sigma and Product Quality Improvement is provided in Module 3.