Let a single letter such as G represent a datum at the corresponding vertex of the
prism in Fig. 1. Let a two-letter combination such as CD represent a number at the
midpoint of the corresponding edge of the prism, and let a four-letter combination like
BDIG represent a number at the midpoint of the corresponding side of the prism. Many
operational equations can be used to estimate a datum that is missing from the prismatic
array [4,5]. The previously published equations are invariant under rotation of the data
and they are often invariant under data translation, too.
There is another class of operational equations that are not invariant under data
rotation. This deficiency limits their applications. Nevertheless, those equations are
potentially useful to the experimentalist. They apply when the data can be satisfactorily
approximated by an arbitrary function applied to monotonic numbers. The equations have
the advantage of increased accuracy in this circumstance. They have that advantage
because they are then numerical identities. Two examples help to clarify the preceding
remarks. The example equations are Eqs. (1) and (2).