Conceptually it could be asserted that there exist an ``internal quality''
(produced quality and quality costs) and an ``external quality'' (perceived
quality downstream and in-bound quality upstream); this sub-division into two
categories is not, however, upheld by the PCA, which does not join the
aforementioned four types of quality two by two. Of less importance (as
relative eigenvalue and variance explained) is a fifth component: machine
reliability.