Effects of Continuous Variables (MANOVA: Univariate command)

Usually you name factors but not covariates on the DESIGN subcommand. The linear effects of covariates are removed from the dependent variable before the design is tested. However, the design can include variables that are measured at the interval level and originally named as covariates or as additional dependent variables.

  • Continuous variables on a DESIGN subcommand must be named as dependents or covariates on the MANOVA variable list.
  • Before you can name a continuous variable on a DESIGN subcommand, you must supply an ANALYSIS subcommand that does not name the variable. This action excludes it from the analysis as a dependent variable or covariate and makes it eligible for inclusion on DESIGN.
  • You can use the keyword POOL(varlist) to pool more than one continuous variable into a single effect (provided that the continuous variables are all excluded on an ANALYSIS subcommand). For a single continuous variable, POOL(VAR) is equivalent to VAR.
  • The TO convention in the variable list for POOL refers to the order of continuous variables (dependent variables and covariates) on the original MANOVA variable list, which is not necessarily their order on the active dataset. This use is the only allowable use of the keyword TO on a DESIGN subcommand.
  • You can specify interaction terms between factors and continuous variables. If FAC is a factor and COV is a covariate that has been omitted from an ANALYSIS subcommand, FAC BY COV is a valid specification on a DESIGN statement.
  • You cannot specify an interaction between two continuous variables. Use the COMPUTE command to create a variable representing the interaction prior to MANOVA.

Example

*  This example tests whether the regression of the dependent 
   variable Y on the two variables X1 and X2 is the same across
   all the categories of the factors AGE and TREATMNT.

MANOVA Y BY AGE(1,5) TREATMNT(1,3) WITH X1, X2
  /ANALYSIS = Y
  /DESIGN = POOL(X1,X2),
            AGE, TREATMNT, AGE BY TREATMNT,
            POOL(X1,X2) BY AGE + POOL(X1,X2) BY TREATMNT
              + POOL(X1,X2) BY AGE BY TREATMNT.
  • ANALYSIS excludes X1 and X2 from the standard treatment of covariates so that they can be used in the design.
  • DESIGN includes five terms. POOL(X1,X2), the overall regression of the dependent variable on X1 and X2, is entered first, followed by the two factors and their interaction.
  • The last term is the test for equal regressions. It consists of three factor-by-continuous-variable interactions pooled together. POOL(X1,X2) BY AGE is the interaction between AGE and the combined effect of the continuous variables X1 and X2. It is combined with similar interactions between TREATMNT and the continuous variables and between the AGE by TREATMNT interaction and the continuous variables.
  • If the last term is not statistically significant, there is no evidence that the regression of Y on X1 and X2 is different across any combination of the categories of AGE and TREATMNT.