  The suitability of a mathematical-model Y = f({Xi}) in serving a purpose
whatsoever (should be preset by the function f specific input-to-output
variation-rates, i.e.) can be judged beforehand. We thus evaluate here the two
apparently similar models: YA = fA(SRi,WRi) = (SRi/WRi) and: YD = fd(SRi,WRi) =
([SRi,WRi] - 1) = (YA - 1), with SRi and WRi representing certain
measurable-variables (e.g. the sample S and the working-lab-reference W
specific ith-isotopic-abundance-ratios, respectively, for a case as the isotope
ratio mass spectrometry (IRMS)). The idea is to ascertain whether fD should
represent a better model than fA, specifically, for the well-known IRMS
evaluation.
  The study clarifies that fA and fD should really represent different
model-families. For example, the possible variation, eA, of an absolute
estimate as the yA (and/ or the risk of running a machine on the basis of the
measurement-model fA) should be dictated by the possible
Ri-measurement-variations (u_S and u_W) only: eA = (u_S + u_W); i.e., at worst:
eA = 2ui. However, the variation, eD, of the corresponding differential (i.e.
YD) estimate yd should largely be decided by SRi and WRi values: ed = 2(|m_i |x
u_i) = (|m_i | x eA); with: mi = (SRi/[SRi - WRi]).
  Thus, any IRMS measurement (i.e. for which |SRi - WRi| is nearly zero is a
requirement) should signify that |mi| tends to infinity. Clearly, yD should be
less accurate than yA, and/ or even turn out to be highly erroneous (eD tends
to infinity). Nevertheless, the evaluation as the absolute yA, and hence as the
sample isotopic ratio Sri, is shown to be equivalent to our previously reported
finding that the conversion of a D-estimate (here, yD) into Sri should help to
improve the achievable output-accuracy and -comparability.
