Defects are the undesirable results of an error in a process. In most cases, this shows up as a product or service not conforming to a specification.
Defects are often expressed as either yield of good parts, such as a 95% yield (meaning a 5% defect rate), or as Defects per Million Opportunities (DPMO).
In the 5% defect example, the DPMO would likely NOT be 50,000 (5%). This is because in this calculation, there may be multiple opportunities for a failure on each unit. The more opportunities for failure per unit, the lower the DPMO would be with the same number of defects. The downside is that multiple defects on the same unit count multiple times.
Detection is the act/art of identifying either errors or defects. Better detection reduces the risk of ‘escapes’ which are defects that make it through to the customer. These affect external quality. Internal quality issues (defects that are caught before the product makes it to the customer) affect cost and delivery time, but do not affect corporate reputation or perception of quality.
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Defects are often defined far too restrictively. Have you ever received a product that didn’t quite measure up to your standards, but probably passed the quality check from the company? As an example, in my car, there is a switch that has what I would characterize as a ‘gritty’ feel to it, but it works. I consider it a defect, but the car company probably does not.
Another very significant point is that I have yet to see a defect that could not be traced back to an error in a process somewhere. The process might be from a supplier, but somewhere, something failed, causing the issue. (Even if it was a machine that failed, there is still a process to maintain it.)
A final point on defects: Calling them by another name doesn’t change what they are. Issue. Non-conforming part. Out of spec. Out of tolerance. Regardless of what you call defects, poor quality is one of the fastest ways to drive customers away.
Pay attention to the number of problems that are fixed on the fly with no record of the correction. It happens more often than you probably realize. This will help you get a better sense for internal quality, and the worse internal quality is, the more likely your customers are to experience defect escapes.