SPSS® 12.0 Command Syntax Reference

1068 NOMREG
B are factors. The expression A WITHIN B is equivalent to A(B). Factors inside a nested
effect must be distinct. Expressions like A(A) and A(B*A) are invalid.
• Multiple-level nesting is supported. 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 model, enter the name of the covariate on the MODEL
subcommand.
• Covariates can be connected, but not nested, using the keyword BY or the asterisk (*)
operator. For example, X*X is the product of X and itself. This is equivalent to a covariate
whose values are the square of those of X. However, X(Y) is invalid.
• Factor and covariate effects can be connected in many ways. No effects can be nested
within a covariate effect. Suppose A and B are factors, and X and Y are covariates.
Examples of valid combination of factor and covariate effects are A*X, A*B*X, X(A),
X(A*B), X*A(B), X*Y(A*B), and A*B*X*Y.
• A stepwise method can be specified by following the model effects with a vertical bar (|),
a stepwise method keyword, an equals sign (=), and a list of variables (or interactions or
nested effects) for which the method is to be used.
• If a stepwise method is specified, then the TEST subcommand is ignored.
• If a stepwise method is specified, then it begins with the results of the model defined on
the left side of the MODEL subcommand.
• If a stepwise method is specified, but no effects are specified on the left side of the MODEL
subcommand, then the initial model contains the intercept only (if INTERCEPT = IN-
CLUDE) or the initial model is the null model (if INTERCEPT = EXCLUDE).
• The intercept cannot be specified as an effect in the stepwise method option.
• For all stepwise methods, if two effects have tied significance levels, then the removal or
entry is performed on the effect specified first. For example, if the right side of the MOD-
EL subcommand specifies FORWARD A*B A(B), where A*B and A(B) have the same significance
level less than PIN, then A*B is entered because it is specified first.
The available stepwise method keywords are:
BACKWARD Backward elimination. As a first step, the variables (or interaction effects or
nested effects) specified on BACKWARD are entered into the model together
and are tested for removal one by one. The variable with the largest significance
level of the likelihood-ratio statistic, provided that the value is larger
than POUT, is removed, and the model is re-estimated. This process continues
until no more variables meet the removal criterion, or when the current
model is the same as a previous model.