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If you compare the sigma value on this site with other values published in most six sigma literature, it's important to know that a 1.5 σ "shift" is usually added. For example, if you see a sigma value of 1.08σ on measuringusability.com and want to compare it to other sources then add 1.5. The resulting process capability will be 2.58 σ. The 1.5σ shift is

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Six Sigma History and Overview Six Sigma was started in manufacturing for processes that are duplicated thousands and millions of times in something like the placement of a spot weld on sheet metal on a power turbine and being able to predict the failure rate of that part’s weld. If you can measure and control the variability in the manufacturing of this metal, you can

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The benefit of using a z-score in usability metrics was explained in "What's a Z-Score and why use it in Usability Testing?" this article discusses different ways of calculating a z-score. The short answer is: It depends on your data and what you're looking for. If you've encountered the z-score in a statistics book you usually get some formula like: The above formula is for

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The discerning usability analyst should employ a mix of both qualitative and quantitative methods when discovering usability problems. The risks of relying heavily on a qualitative approach can lead to a severe misdiagnosis especially when usability problems are difficult to detect. This article is a response to Nielsen’s "The Risk of Quantitative Studies" and shows how the problems voters had with the “butterfly-ballot” in the

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