Variable and constant performance errors within a group of individuals.

The correct individual single score for a within-S series of errors e about the target (for k trials) is E= √∑e(2) /k= √V(2) +C(2); the commonly used absolute error AE under-represents or deletes the variable error component V. In correlational analysis, the constant error score should be C absolute and V should be used unsquared in order to avoid curvilinearity; in general, r patterns across Ss are not predictable from within-Ss relations. While V and C are necessarily independent within Ss, they usually exhibit substantial correlation across Ss; evaluation of the role of each is sometimes important. Linearity of regression is demanded; it, rather than non-skewness, is shown to be the important assumption in using r. If relations involving algebraic C are of interest, the correlation index may be required because of U-shaped regression. Several common statistical misinterpretations are discussed.