Because a manufacturing process that produces virtually no products
out of specification tends to be very expensive, there is often an
acceptance criterion allowing a small amount of deviation from
specification. This criterion is commonly defined as the lowest acceptable
level of a process capability index
1
(PCI), the highest number of defect
parts per million (PPM) or sometimes as maximum defects per million
opportunities (DPMO) (Wu, et al., 2009). The aim of using this kind of
criteria is to prohibit poor production process performance in relation to
the requirement, preventing adherent high quality deficiency costs.
PCIs is often used to set a limit for the lowest acceptable process
capability, but has not been used for limiting the highest acceptable
process capability. The aim of a higher limit should be to prevent
excessive manufacturing cost due to overqualified manufacturing
resources; this is in contrast to the commonly used lower limit which
aims to save the customer from getting a product out of specification.
Even though a useful higher limit of capability index is not defined in the
literature, the assumption here is that the process planner must try to
obtain a balance between required product performance and the
economic efforts put into the manufacturing process.