When we discuss or mention Six Sigma, we almost always hear about getting defect rates down to 3.4 ppm. Consider a light bulb. How would you like to have a defect rate of 3.4 ppm only to find out that the bulb lasted just 20 hours before it burned out? This isn’t good, even though the Cpk was 1.50. The definition of quality relates to fitness for use, meeting the specification and the life of the product. That is, how long will it satisfy the customers’ needs? That’s reliability.

There are two major quality topics that concern the element of time. They’re statistical process control (SPC), which is how the process performs over time; and reliability, which is how long the product lasts. Reliability is the probability of mission success over a defined period of time, under a defined operating environment. Today it’s possible to substitute the word "cycles" in place of the word "time." But before defining reliability, it’s important to define failure. In the light bulb example, it’s simple: it either lights or it doesn’t. For more complex systems, the definition of failure isn’t as straightforward. What would be the definition of failure for human beings, for example?

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