The mathematics used for Quality Enginearing is not the one based on statistical mathematics but the one called quadratic expansion.
The degree of freedom is defined as the rank of adjoined matrix of each quadratic term of the expansion. The reason that the writer used the name of degree of freedom is also explaned in this article. Quadratic expansion is related with Hermitian expansion in a complex domain in mathematical form and is related with power spectral expansion from the viewpoint of communication theory.
