## What Is the Logic Behind a One Tail Defect, Or Is it That This Difference Is Negligible Beyond 3.5 Sigma?

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Of considerable interest, this concept often mystifies newcomers to field of Six Sigma. As you already surmised, the referenced charts rely upon the assumption of a unilateral specification (single-sided performance requirement). From a defect accounting point-of-view, it should be fairly obvious that a unilateral (one-sided) specification must be treated as a single defect category. However, a bilateral (two-sided) […]

## If a Project Doesn’t Meet the Six Sigma Level (as Calculated on the Sigma Calculator on This Site), Is it a Six Sigma Project?

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Seems to me that you have described a traditional quality improvement project. Based on the elements of your question, it is my understanding that: 1) Each “drawing feature” constitutes a “defect opportunity.” 2) The customer’s minimum expectation was a quality level of .99, per feature. 3) The pre-project quality was found to .995, per feature […]

## How Are Defects and Opportunities Determined for CTQs?

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To begin, recall that a critical-to-quality characteristic (CTQ) is a vital product design feature (e.g., the strength of a material).  Naturally, we recognize that the product (per se) does not necessarily have to be a piece of hardware – it can be anything (e.g., a software program, a delivered service, a financial transaction, a social […]

## Can You Explain Shifting of 1.5 Sigma in Calculating Sigma Level, Including Physical Meaning of Shifting?

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Over time, there has been very fine debate (both positive and negative) surrounding the 1.5 sigma shift.  As such, the on-going discussion well serves the need to “keep the idea alive,” so to speak.  To this end, I have recently completed a small book on the subject, soon to be available on iSixSigma.  The title […]

## Why Is a Process Shift of 1.5 Sigma Taken as a Standard for Calculations?

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Right up front in our discussion, let us recognize that the 1.5 sigma shift can be attributable solely to the influence of random error. In this context, the 1.5 sigma shift is a statistically based correction for scientifically compensating or otherwise adjusting a postulated model of instantaneous reproducibility (short-term capability) for the inevitable consequences associated […]

## If Our Vendors Have a Quality Level of 3.5 Sigma, Up to What Level of Sigma Can We Achieve?

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To open our discussion, we acknowledge that not all product (or service) opportunities should be Six Sigma. In the total scheme of things, some are not destined to be critical – to anything. For example, in a garden supply store we can find many different types of tools.  More specifically, let us consider an ordinary hand […]

## How Can I Determine If My Rolled Throughput Yield (RTY) Is Acceptable?

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Let us postulate that a certain process is comprised of three progressive steps, say A, B, and C, respectfully. Further, we note the observed defects-per-unit (dpu) at the first step to be dpu.A = .10. At the second step, dpu.B = .05. The third step was found to be dpu.C = .008. Of course, we acknowledge dpu as a […]

## Can I Extrapolate 3.4 Defects Per Million to 34 Defects Per 10 Million or 3400 Defects Per Billion?

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To help you better understand the nature of this question; let us consider a very simple example.  Let us suppose that we go to the store – looking for fresh apples.  Once there, we notice a sign that reads “3 apples for \$1.00.”  Based on this sign, we ask the storekeeper: “If I want to buy […]

## My Boss Calculates Our Process Sigma at 3 Sigma, While I Calculate It at 5 Sigma. Who Is Correct?

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Perhaps you are both in error (as related to your Sigma calculations.) Do not be discouraged by such debate, as it is quite common among those new to Six Sigma. However, do be aware of a phenomenon I call “denominator management.” This colorful term is used to describe the practice of inflating or otherwise distorting the denominator […]

## What Is the Correct Manner to Count Defects and Opportunities, Used for Calculating Process Sigma Value?

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As you likely know, the issue of opportunity counting is somewhat illusive. It is laced with many misconceptions. The seeming intuitive nature of a “defect opportunity” runs wider and deeper than what one would naturally expect. To help us better understand this concept (and some of the related pitfalls), we will discuss the classic quality metric […]

## How Can One Calculate the Sigma Level of a System at a Business Level (For Example, a Marketing System)?

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Consider the case where a particular system is comprised of three subordinate elements. In this case, we will say the first-time yield (quality confidence) of each element is known to be .85, .90, and .95, respectively. The cross product would then be given as .85 x .90 x .95 = .612, or about 61.2 percent.  This value […]

## The Sigma Conversion Table Displays Yields That Don’t Match My Understanding of USL/LSL for the Process. Could You Please Explain Why There Is a Discrepancy?

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To facilitate an answer to your question, consider the following table. The first column is the given measure of process capability (provided in the form of Z).  The second column is the corresponding short-term yield of the first column (based on a one-sided specification). Of course, the third column is the associated long-term yield, also predicated on […]

## Why Is 1.5 Subtracted From the Short-term Estimate of Capability (Z.st) to Get the Long-term Estimate of Capability (Z.lt)?

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The capability of a process has two distinct but interrelated dimensions. First, there is short-term capability, or simply Z.st. Second, we have the dimension long-term capability, or just Z.lt. Finally, we note the contrast Z.shift = Z.st – Z.lt. By rearrangement, we assuredly recognize that Z.st = Z.lt + Z.shift and Z.lt = Z.st – Z.shift. So as to better […]

## Is it Possible to Determine a Corporate-wide Sigma Level? How Can the Sigma Level be Calculated?

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So as to establish a corporate-level sigma value (per se), it is generally advisable to follow an established analytical model. Of course, any such model must be conducive to the hierarchical aggregation (pooling) of capability information and data. Generally speaking, the more common approaches for realizing this aim are inclusive of, but not limited to: 1) the […]

## Is 7 Sigma Possible?

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Don’t know, never been there – I’m still striving for 5-sigma (at least on a personal level). Given where the world is right now, many followers of Six Sigma (including myself) would say that a capability of 7-sigma is pessimistically possible, but not pragmatically probable. Maybe when I hit a 5-sigma level of capability, I can […]

## How Do I Determine Defects Per Million (DPM) for Sigma Values Greater Than 6 (3.4 DPM)?

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Once is a great while, there is a need to estimate or otherwise approximate the tail probability of an extreme standard normal deviate – such as Z = 7, Z = 10, or even higher.  For example, certain types of reliability problems rely on such assessments. But as many already know, most published tables of […]

## How Is the Defects Per Million Opportunities (DPMO) to Sigma Level and Yield Conversion Chart Determined?

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We must understand the sigma of a process is merely an equivalent unilateral figure-of-merit. As such, it is always regarded (and reported) as a short-term measure of capability. The metric can be directly computed when continuous data is at hand, or it can be statistically synthesized from discrete data. In the latter context, a reported sigma is only […]