In any production, facility, no matter how perfect the operations are defective products will intermittently appear in the output. There are six sources of quality problems in manufactured goods. Design perturbation refers to problems in which the design is inherently sound but certain key parameters need to be further adjusted to improve quality. Design to manufacturing interface refers to problems in which the design is sound, but from a manufacturing point of view the potential for defects arising exist.
Bad material refers to problems existing in materials obtained from outside vendors.
Manufacturing perturbation refers to problems in which some of the production processes, including inspection and monitoring, need modifications to further improve the yield. Bad manufacturing refers to problems that stem from a lack of skill, training, discipline, or inadequacy in terms of processing capabilities. The first two sources determine the design quality of the product. The customer defines design quality as the utility of the product as Perceived. This utility is measured in terms of Functionality Usability Reliability Performance Serviceability Availability Price.