MODEL Subcommand (CSCOXREG command)

The MODEL subcommand is used to specify the effects to be included in the model.

  • Specify a list of terms to be included in the model, separated by spaces or commas.
  • If the MODEL subcommand is not specified, CSCOXREG uses a model that includes main effects for any factors, and any covariates, in the order specified on the variable list.
  • To include a term for the main effect of a factor, enter the name of the factor.
  • To include a term for an interaction among factors, use the keyword BY or the asterisk (*) to join the factors involved in the interaction. For example, A*B means a two-way interaction effect of A and B, where A and B are factors. A*A is not allowed because factors inside an interaction effect must be distinct.
  • To include a term for nesting one factor within another, use a pair of parentheses. For example, A(B) means that A is nested within B. A(A) is not allowed because factors inside a nested effect must be distinct.
  • Multiple nesting is allowed. For example, A(B(C)) means that B is nested within C, and A is nested within B(C). When more than one pair of parentheses is present, each pair of parentheses must be enclosed or nested within another pair of parentheses. Thus, A(B)(C) is not valid.
  • Nesting within an interaction effect is valid. For example, A(B*C) means that A is nested within B*C.
  • Interactions among nested effects are allowed. The correct syntax is the interaction followed by the common nested effect inside the parentheses. For example, interaction between A and B within levels of C should be specified as A*B(C) instead of A(C)*B(C).
  • To include a covariate term in the design, enter the name of the covariate.
  • Covariates can be connected, but not nested, through the * operator or using the keyword BY to form another covariate effect. Interactions among covariates such as X1*X1 and X1*X2 are valid, but X1(X2) is not.
  • Factor and covariate effects can be connected in various ways except that no effects can be nested within a covariate effect. Suppose A and B are factors and X1 and X2 are covariates, examples of valid combinations of factor and covariate effects are A*X1, A*B*X1, X1(A), X1(A*B), X1*A(B), X1*X2(A*B), and A*B*X1*X2.