Quadratic and cubic equations for four-point rectangles are new. Second- and
third-degree polynomial models of four-point rectangular surfaces can predict
spurious curvature effects, including unwarranted inflections and extrema. This
being recognized, they are potential improvements on the bilinear equation for
four-point rectangular data arrays. One measure of the improvement is the sumof-
squares-of-deviations test. Table 1 indicates that both four-point quadraticequation
models exhibit lower sums than the bilinear-equation model. No method
is known that predicts which quadratic equation will be the better choice for
laboratory data. The four-point cubic-equation model typically renders even lower
sums as illustrated by the table. Table 1 lists results obtained on monotonicincreasing
or monotonic-decreasing data. The illustrated methods do not require
monotonic data. Preferred models are determined by laboratory experience.