Zero Defects: What Does It Achieve? What Does It Mean?

The definition for Six Sigma was clear from the beginning – 3.4 defects per million opportunities (DPMO), allowing for a 1.5-sigma process shift. But the definition for zero defects is not so clear. Perhaps zero defects refers to the domain beyond 3.4 DPMO. Or perhaps it refers to designing defects out of the process or product, so that – theoretically at least – a company can consistently manufacture a defect-free product.

There is value in trying to understand the meaning and purpose of this oft-used term, and whether its use is the best approach in a Six Sigma environment of continuous improvement.

Possible Pitfalls of Pushing Zero Defects

Quality guru W. Edwards Deming believed that slogans and programs such as zero defects are usually counterproductive. D.C. Montgomery, author of the book Introduction to Statistical Quality Control, agrees, commenting that these programs typically do not drive the “use of proper statistical and engineering tools into the right places of the organization,” and they “devote far too little attention to variability reduction.” In other words, the use of slogans such as zero defects to spur quality may lead to a de-emphasis of the tried-and-true tools and culture associated with successful continuous improvement.

But can a mere slogan actually discourage the successful implementation of proven Six Sigma continuous improvement methodologies? This can best be answered by considering the expectations, the conflicts and the different levels of understanding surrounding the term zero defects.

Literally zero defects corresponds to a defect level of infinity sigma, which most practitioners will admit is not possible. And yet an enthusiastically institutionalized zero defects program may unfortunately promote the belief and expectation that true zero can and should be achieved. This is evidenced by several phrases that quality professionals may have heard spoken – or at least heavily implied – by business strategists:

  • “All defects are the same, since all defects are bad…”
  • “There is no such thing as a benign defect.”
  • “If we can get rid of the defects, then we can get rid of the testing.”

These expectations are worth examination.

Statements That Do Not Align with Facts

In fact, all defects are not equal. Defects, depending on their size and type, have different probabilities of impacting the finished product. And these probabilities depend on the technology. In fact, the impact probability of a particular defect may vary within the technology – that is, at the stage or layer in which it occurs. When it comes to the practical definition of a defect, “bad” is a relative term. Many defects are simply neutral. They are never good, but – again, depending on the technology – they may cause no harm either. If all defects are considered bad, then prioritization is difficult.

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It is the role of statistically minded scientists and engineers to classify defects and their potential impact, based on data and engineering judgment. This allows them to systematically reduce defect levels in a prioritized fashion, starting with the worst and progressing toward the more benign. Without this kind of problem-solving prioritization, progress may be slow and confused – perhaps even at a standstill. The ability to prioritize is absolutely necessary in the continuous improvement process.

The statement that if fewer defects are produced, then less inspection will be required is incorrect. Actually, the opposite is true. A higher level and sophistication of testing is required to detect a smaller level of defects. The plot in Figure 1, derived from a cumulative binomial distribution (pass/fail inspection) shows how the sample size increases exponentially as the prevalence of a defective unit decreases. The particular curve in Figure 1 corresponds to a probability of detection of 95 percent. In other words, if a defect is present at the indicated level (x-axis), there is a 95 percent probability that at least one failed unit will be detected using the sample size indicated on the y-axis.

Figure 1: Sample Size Versus Probability of Failure

Figure 1: Sample Size Versus Probability of Failure

A more intuitive example is: If a shoebox full of needles is mixed into a haystack, only a portion of the haystack will have to be moved before the presence of needles is detected. If there is only one needle in the haystack, every straw may have to be moved before it is found, assuming it is not missed entirely.

This is really the misunderstanding that drives the inappropriate application of a zero defects policy to multiple points along the supply chain (Figure 2). It may be thought that producing zero or near-zero defects at each point will lead to reduced or eliminated inspection/testing prior to shipment to the end-customer. But for zero defects to approach reality, the inspection/testing must remain the same or increase at the final inspection point. If zero is truly the goal, then 100 percent sampling at the “escape” point is required, regardless of defect levels. This implies, then, that any zero defect inspections prior to the escape point may be non-value-added.

Figure 2: A High-level Flow of Serial Product Manufacture, Across Supplier and Customer Boundaries

Figure 2: A High-level Flow of Serial Product Manufacture, Across Supplier and Customer Boundaries

Ideally suppliers need to produce the highest quality output possible, in order to maximize yield and minimize costs which ultimately benefits both the supplier and the customer. But a zero defects policy does not provide this motivation to suppliers. When the goal of zero defects is applied to multiple interim points along the supply chain, the undesired effects of increased costs and lower yields are encouraged. The increased costs come from increased tests, inspections and cycle time. The lower yields are likely because of a higher rate of “false fails” (type 1 errors) as the suppliers apply increasingly stringent criteria in an attempt to eliminate potential failures at the customer’s incoming test/inspection. In other words, in an effort to eliminate even the smallest possibility of customer incoming test failures, good product may be scrapped to overly stringent criteria.

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Negative Impact on Workforce and Supply Chain

A focus on zero defects may be stifling to a discussion of continuous improvement, and may lead to frustration and non-productivity. To the general workforce, it may be a demoralizing concept. While everyone understands that continuous defect reduction is critical and necessary, most people understand, intuitively at least, that true zero is unachievable. Always striving for an unachievable goal may eventually de-motivate even the most optimistic of employees, particularly if they are frequently told that their defect level is unacceptable – because it is not zero.

For a company’s suppliers, continuing to add tests and inspections in an effort to comply with zero defects (perhaps at their customer’s demand) may eventually drive them out of business. Thus, while continuous improvement is applicable to everyone, zero defects can or should only be applied to the final supplier, rather than at interim points along the supply chain. Attempting to do the latter may eventually put one or more of the suppliers in jeopardy. If a supplier critical to the company were to fail, the company’s supply chain might collapse, which might eventually put the company out of business too

Finally, it should be realized that the inspections and tests themselves (however careful and precise they are) have a finite probability of actually causing a defect. This concept is somewhat akin to the uncertainty principle: “We may significantly modify what we are trying to measure simply by making the measurement.”

Conclusion: Strive to Be Better and Better, Not Perfect

Since the slogan zero defects implies immediate compliance to a defect-free standard, it may not leave time for the continuous improvement process to occur. In fact, it may even slow down the continuous improvement process because of the massive resources that inspected-in quality entails.

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Zero defects is a message that can carry with it confusion and misinterpretation, mixed with technical impracticality. It may be appropriate that the idea of zero defects be replaced with a policy of zero escapes, since the latter has limited interpretation. As a company is doing all it can to improve the product and business using continuous improvement techniques, it also needs to consider what it can do to prevent a random, low-level defect from reaching the final customer. In this regard, zero escapes of defects may be a complimentary activity to continuous improvement.

A logical strategy is to employ continuous improvement methodologies everywhere in the business and manufacturing process to improve quality and yield, and reduce cycle time and costs. Then, at the point of shipping the final product to the final customer, employ a zero escapes methodology to help ensure that a randomly defective unit does not reach its final application. The tools and techniques developed and employed at this final gate should be arrived at through a team effort of the various suppliers and interim customers. Expecting individual suppliers in the supply chain to produce zero defects, in an effort to eliminate or minimize the final gate, is likely to be an impractical strategy.

Quality professionals already have specific, descriptive methodologies that are aimed at achieving the same goals as zero defects. Here are but some of the methodologies already in use and being developed to minimize the defects in the end product:

  • Design for manufacturability (DFM)
  • Design for yield (DFY)
  • Design for test (DFT) – “DFM: Worlds Collide, Then Cooperate” by L. Peters in Semiconductor International, June 1, 2005.
  • Robust design

It is probably best to not encourage the use of somewhat ambiguous terminology in the place of well-defined and meaningful methodologies such as these.

The concept of continuous improvement is intuitive. It makes sense to always strive for a better process or product, to reduce costs, satisfy customers and gain market share. Absolute perfection can never be achieved, but an organization can move closer and closer with good statistical and engineering practices.

Comments 19

  1. Karen

    This articles seems to totally miss the point of the zero defects concept. Phil Crosby was the proponent and repeatedly had to fight with people who failed to understand the concept. The concept is simple and has nothing to do with statistics and far less to do with appraisal and testing. It is a quality mindset / management policy which says ” I will manufacture the product I set out to manufacture” or “I will provide the service I claim to provide” or even more simply – I will conform with MY OWN requirements, after having define these. Instead of adopting this mindset, companies spend millions going “lean” or “six sigma” in order to justify producing or providing non-conforming, often harmful products or services.

  2. Paulo Cesar Ferreira Franco

    Very Good! Congrats! A very interesting approach to Zero Defect concept.

  3. Waggle

    A defect may be lot worse than a mistake. A defective tyre may kill you while a tyre with a spelling mistake will not. Both are nonconformities but a nonconformity must have the potential to adversely affect the performance of the product to be considered a defect.

    So, when we say zero defects are we really saying zero nonconformities?

    The biggest improvement I saw arising from Crosby’s zero defects teaching was that designers stopped issuing specifications that were impossible to fulfill. Mind you, this did lead to food specs openly allowing small amounts of rat feces in food!

  4. Roger Ellis

    The author started out in the first sentence of this article by confusing Six Sigma (the process improvement methodology) with sigma level of quality (a metric that defines a sigma level of six as 3.4 DPMO with a 1.5 standard deviation mean shift). The mean shift itself is controversial. Without the mean shift, the DPMO number is .002 instead of 3.4 – very, very close to zero defects.

    The author went on to make the assumption that the only way to achieve zero defects is to perform more inspection. This is a flawed assumption. Dr. Deming’s third point is “”Cease dependence on inspection to achieve quality. Eliminate the need for massive inspection by building quality into the product in the first place.” The route to quality improvement is finding and eliminating the root cause of problems, not adding more inspection.

    I agree with the author that not all defects are created equal. However, a defect is defined as something that does not meet customer expectations. How can a defect be “neutral” or “cause no harm” if the customer is less satisfied?

    Zero defects as a goal can be and is applied to many types of defects. Consider the airline industry where a crash is considered to be a defect. The goal is zero crashes. The industry strives to reach this goal by systematically investigating all crashes and working to eliminate their root causes (not by adding more inspection). The domestic airline industry in the United States has in fact recorded entire years with zero crashes.

    In the healthcare industry, performing the wrong surgery on a patient is a defect. No level of defects is acceptable, and no amount of inspection after the fact will improve quality. I mentored a Black Belt student project recently that was targeted at eliminating the root causes of wrong patient/wrong side/wrong procedure surgery across the state of Florida.

    Zero defects may or may not ever be achieved, but it is always the goal.

    Roger C. Ellis, Six Sigma Master Black Belt

  5. Tony

    “The definition for Six Sigma was clear from the beginning – 3.4 defects per million opportunities (DPMO), allowing for a 1.5-sigma process shift.”
    Yes, Six Sigma was based on utter nonsense right from the beginning. The 3.4 was based on a fraud followed by pathetic attempts to cover it up. Millions of dollars were poured down the drain over the farcical 1.5 sigma drift in the claimed “long term” of Harry’s 24 hours.

  6. Pascal

    zero defect could be a target but never an objective
    you need to balance prevention cost and defect cost

  7. V. Rosario

    Found an interesting spin on “Zero Defects Culture”…

  8. DC

    Great article, thank you. I see this confusion and ignorance very regularly amongst managers of knowledge workers in support / service roles. They use the jargon to beat up their staff, measure nothing, inspect without any statistical basis (who will I pick on today approach). The results are high staff turnover due to frustration, staff who give up on quality as they know that their supervisor or manager will find a defect no matter how diligent they are (and you are right, defects have different impacts on the products or services), all of which reinforces the supervisors view that their team needs to take a zero defects approach. Reversing this thinking can be very challenging.

  9. bob

    “We will chase perfection, and we will chase it relentlessly, knowing all the while we can never attain it. But along the way, we shall catch excellence.” by Vince Lombardi Jr.
    40 years of manufacturing experience and still striving to achieve the big ZERO.
    Good article, makes us think!

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