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Zero defects and robust quality are two quality philosophies used to help U.S. companies compete in the global marketplace. An important measure in any quality philosophy is the cost of product variability.
Although variation among products may affect the consumers' perceptions of quality, the importance of consistency to a producer depends on the quality philosophy it follows. For example, with a zero defects philosophy the only cost attributed to variation occurs when products fall outside the specification limits. Thus, the cost of variation depends on whether the product can be reworked, if there are constraints on production, and the distance between product specification limits.
On the other hand, a robust quality program claims that any variation is undesirable and causes costs to be incurred by the manufacturer, consumer, or society. An estimate of these costs is provided by the quality loss function.
TWO PHILOSOPHIES
The term "quality" has many different meanings depending on the context used. When quality is used to describe products, it may mean "conforming to specifications" or "fitness for use." Although these two definitions may mean the same thing to many people, they have different implications for how quality is evaluated and the importance of product variability.
The philosophy underlying the "conformance to specifications" definition generally leads to a zero defects approach to quality while the "fitness for use" philosophy leads to a robust quality approach.(1)
The role of product variability in evaluating quality depends on whether the company has adopted a zero defects or a robust quality philosophy. With a zero defects philosophy, the allowable variation is defined by specification limits, and any variation within those limits is acceptable. According to the robust quality philosophy, any variation from a target value represents a condition that is less than ideal, with potential economic consequences. Thus, robustness is the result of meeting exact targets consistently--not from always staying within tolerances.
In turn, the cost of variation also depends on the philosophy. Generally, companies that adopt a zero defects philosophy will not attribute any cost to variability if the product is within specification limits. But the robust quality proponent will assign a cost to variability whenever a product varies from a target. Companies following a robust quality philosophy believe any variation from target results...