Multivariate Analysis: Correspondence Analysis

You can use the Roles tab to specify a weight variable or supplementary variables for the analysis. (See Figure 31.10.)

A weight variable is a numeric variable that represents category frequencies. In the absence of a weight variable, each observation contributes a value of 1 to the frequency count for its category. That is, each observation represents one subject. When you specify a weight variable, each observation contributes the value of the weighting variable for that observation. For example, a weight of 3 means that the observation represents three subjects.

Supplementary variables are displayed as points in the configuration plot, but these variables are not used in computing the correspondence analysis. In other words, a supplementary variable is projected onto the principal coordinate directions, but it is not used to compute the principal coordinates.

Note: In the CORRESP procedure, supplementary variables must be listed in the TABLE statement in addition to being listed in the SUPPLEMENTARY statement. In SAS/IML Studio, you should not specify supplementary variables on the Variables tab.

As an example of using supplementary variables, suppose you use the Variable Transformation Wizard to create a nominal variable that indicates whether a company is profitable. You can display the levels of this variable in the configuration plot by adding the variable to a supplementary variable list, as shown in Figure 31.10.