ABSTRACT
Once a classifier S(X) has been constructed from the vector X of primary variables and is in use for allocating individuals to one or other of the populations N and P, it frequently transpires that a further variable or set of variables will provide useful classificatory information which will modify the behavior of the classifier in one or both of the populations. Such additional variables are termed covariates, and for maximum benefit should clearly be incorporated into any analyses involving the classifier. Let us denote a set of c covariates by the vector Z, recognizing that in many practical applications we may have just one covariate Z in which case c = 1.
