Modern manufacturing systems, based on the concepts of interchangeable manufacture, require that each part of an assembly going into a final product be made to definite size, shape, and finish specification. The mass production of both consumer and producer goods relies on interchangeability, and interchangeability requires fabrication to exacting dimensions and close tolerances. The quality assurance function within a manufacturing organization, however it may be structured, is charged with the responsibility of maintaining product quality consistent with those requirements. Statistical process control (SPC) or statistical quality assurance methods and they involve the integration of quality control in each stage of producing the product. The tolerance that should be assigned to a dimension is that which gives an economic balance between quality and cost. Tolerances and clearances are specified to assure that mating parts meet the functional requirements and specifications.