modSearch is used internally by speff to construct an optimal model for
prediction of the study endpoint or estimation of the missingness mechanism.modSearch(formula, x, y, endpoint, method, optimal)~ operator, and the linear
predictor on the right.quantitative" or "dichotomous".exhaustive", "forward", or
"backward".cp" for
Mallow's Cp, "bic" for BIC, and "rsq" for R-squared.speff