FIXED Subcommand (MIXED command)

The FIXED subcommand specifies the fixed effects in the mixed model.

  • Specify a list of terms to be included in the model, separated by commas or spaces.
  • The intercept term is included by default.
  • The default model is generated if the FIXED subcommand is omitted or empty. The default model consists of only the intercept term (if included).
  • To explicitly include the intercept term, specify the keyword INTERCEPT on the FIXED subcommand. The INTERCEPT term must be specified first on the FIXED subcommand.
  • To include a main-effect term, enter the name of the factor on the FIXED subcommand.
  • To include an interaction-effect term among factors, use the keyword BY or the asterisk (*) to connect factors involved in the interaction. For example, A*B*C means a three-way interaction effect of the factors A, B, and C. The expression A BY B BY C is equivalent to A*B*C. Factors inside an interaction effect must be distinct. Expressions such as A*C*A and A*A are invalid.
  • To include a nested-effect term, use the keyword WITHIN or a pair of parentheses on the FIXED subcommand. For example, A(B) means that A is nested within B, where A and B are factors. The expression A WITHIN B is equivalent to A(B). Factors inside a nested effect must be distinct. Expressions such as 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 invalid.
  • 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, the 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 FIXED subcommand.
  • Covariates can be connected using the keyword BY or the asterisk (*). For example, X*X is the product of X and itself. This is equivalent to entering a covariate whose values are the squared values of X.
  • Factor and covariate effects can be connected in many ways. Suppose that A and B are factors and X and Y are covariates. Examples of valid combinations 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.
  • No effects can be nested within a covariate effect. Suppose that A and B are factors and X and Y are covariates. The effects A(X), A(B*Y), X(Y), and X(B*Y) are invalid.
  • The following options, which are specific for the fixed effects, can be entered after the effects. Use the vertical bar (|) to precede the options.

NOINT. No intercept. The intercept terms are excluded from the fixed effects.

SSTYPE(n). Type of sum of squares. Specify the methods for partitioning the sums of squares. Specify n = 1 for Type I sum of squares or n = 3 for Type III sum of squares. The default is Type III sum of squares.

Example

MIXED  SCORE BY SCHOOL CLASS WITH AGE PRETEST
  /FIXED = AGE(SCHOOL) AGE*PRETEST(SCHOOL)
  /RANDOM = CLASS.
  • In this example, the fixed-effects design consists of the default INTERCEPT, a nested effect AGE within SCHOOL, and another nested effect of the product of AGE and PRETEST within SCHOOL.

Example

MIXED SCORE BY SCHOOL CLASS
  /FIXED = | NOINT
  /RANDOM = SCHOOL CLASS.
  • In this example, a purely random-effects model is fitted. The random effects are SCHOOL and CLASS. The fixed-effects design is empty because the implicit intercept term is removed by the NOINT keyword.
  • You can explicitly insert the INTERCEPT effect as /FIXED = INTERCEPT | NOINT. But the specification will be identical to /FIXED = | NOINT.