Early
stone tools such as
anvils had no holes and were not designed as
interchangeable parts.
Mass production established processes for the creation of parts and system with identical dimensions and design, but these processes are not uniform and hence some customers were unsatisfied with the result. Quality control separates the act of
testing products to uncover defects from the decision to allow or deny product release, which may be determined by fiscal constraints. For contract work, particularly work awarded by government agencies, quality control issues are among the top reasons for not renewing a contract. The simplest form of quality control was a sketch of the desired item. If the item did not match the sketch, the item was rejected, in a simple
Go/no go procedure. However, manufacturers soon found it was difficult and costly to make parts be exactly like their depiction; hence around 1840 tolerance limits were introduced, wherein a design would function if its parts were measured to be within the limits. Quality was thus precisely defined using devices such as
plug gauges and
ring gauges. However, this did not address the problem of defective items; recycling or disposing of the
waste adds to the cost of production, as does trying to reduce the defect rate. Various methods have been proposed to prioritize quality control issues and determine whether to leave them unaddressed or use
quality assurance techniques to improve and stabilize production. ==Notable approaches==