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# What is six sigma

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The concepts surrounding the drive to Six Sigma quality are essentially those of statistics and probability. In simple language, these concepts boil down to, "How confident can I be that what I planned to happen actually will happen?" Basically, the concept of Six Sigma deals with measuring and improving how close we come to delivering on what we planned to do.

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### What is six sigma

1. 1. What is Six Sigma?The concepts surrounding the drive to Six Sigma quality are essentially those of statistics and probability. In simplelanguage, these concepts boil down to, “How confident can I be that what I planned to happen actually will happen?”Basically, the concept of Six Sigma deals with measuring and improving how close we come to delivering on what weplanned to do.Anything we do varies, even if only slightly, from the plan. Since no result can exactly match our intention, we usuallythink in terms of ranges of acceptability for whatever we plan to do. Those ranges of acceptability (or tolerance limits)respond to the intended use of the product of our labors–the needs and expectations of the customer.Here’s an example. Consider how your tolerance limits might be structured to respond to customer expectations in thesetwo instructions:“Cut two medium potatoes into quarter-inch cubes.” and “Drill and tap two quarter-inch holes in carbon steel brackets.”What would be your range of acceptability–or tolerances–for the value quarter-inch? (Hint: a 5/16” potato cubeprobably would be acceptable; a 5/16”threaded hole probably would not.)Another consideration in your manufactureof potato cubes and holes would be the inherent capability of the way you produce the quarter inch dimension–thecapability of the process. Are you hand-slicing potatoes with a knife or are you using a special slicer with preset blades?Are you drilling holes with a portable drill or are you using a drill press? If we measured enough completed potatocubes and holes, the capabilities of the various processes would speak to us. Their language would be distributioncurves.Distribution curves tell us not only how well our processes have done; they also tell us the probability of what ourprocess will do next. Statisticians group those probabilities in segments of the distribution curve called standarddeviations from the mean. The symbol they use for standard deviation is the lower-case Greek letter sigma.For any process with a standard distribution (something that looks like a bell-shaped curve), the probability is 68.26%that the next value will be within one standard deviation from the mean. The probability is 95.44% that the same nextvalue will fall within two standard deviations. The probability is 99.73% that it will be within three sigma; and 99.994%that it will be within four sigma. Four sigma design specification width Few Defects Few Defects (about 60 DPMO) (about 60 DPMO) -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 Standard distribution curve with mean, sigma values and four sigma tolerances. DPMO = Defect Per Million Opportunities
2. 2. If the range of acceptability, or tolerance limit, for your product is at or outside the four sigma point on the distributioncurve for your process, you are virtually assured of producing acceptable material every time–provided, of course, thatyour process is centered and stays centered on your target value.Unfortunately, even if you can center your process once, it will tend to drift. Experimental data show that mostprocesses that are in control still drift about 1.5 sigma on either side of their center point over time.This means that the real probability of a process with tolerance limits at four sigma, producing acceptable material isactually more like 98.76%, not 99.994%.To reach near-perfect process output, the process capability curve must fit inside the tolerances such that the tolerancesare at or beyond six standard deviations, or Six Sigma, on the distribution curve. That is why we call our goal Six Sigmaquality.Quality makes us strongIn the past, conventional wisdom said that high levels of quality cost more in the long run thanpoorer quality, raising the price you had to ask for your product and making you lesscompetitive. Balancing quality with cost was thought to be the key to economic survival. Thesurprising discovery of companies which initially developed Six Sigma, or world-class, qualityis that the best quality does not cost more. It actually costs less. The reason for this is somethingcalled cost-of-quality. Cost-of-quality is actually the cost of deviating from quality–paying forthings like rework, scrap and warranty claims. Making things right the first time–even if it takesmore effort to get to that level of performance–actually costs much less than creating thenfinding and fixing defects.