# Categorical Principal Components Analysis

The use of Categorical Principal Components Analysis is most appropriate when you want to account for patterns of variation in a single set of variables of mixed optimal scaling levels. This technique attempts to reduce the dimensionality of a set of variables while accounting for as much of the variation as possible. Scale values are assigned to each category of every variable so that these values are optimal with respect to the principal components solution. Objects in the analysis receive component scores based on the quantified data. Plots of the component scores reveal patterns among the objects in the analysis and can reveal unusual objects in the data. The solution of a categorical principal components analysis maximizes the correlations of the object scores with each of the quantified variables for the number of components (dimensions) specified.

An important application of categorical principal components is to examine preference data, in which respondents rank or rate a number of items with respect to preference. In the usual IBM® SPSS® Statistics data configuration, rows are individuals, columns are measurements for the items, and the scores across rows are preference scores (on a 0 to 10 scale, for example), making the data row-conditional. For preference data, you may want to treat the individuals as variables. Using the Transpose procedure, you can transpose the data. The raters become the variables, and all variables are declared ordinal. There is no objection to using more variables than objects in CATPCA.

Relation to other Categories procedures. If all variables are declared multiple nominal, categorical principal components analysis produces an analysis equivalent to a multiple correspondence analysis run on the same variables. Thus, categorical principal components analysis can be seen as a type of multiple correspondence analysis in which some of the variables are declared ordinal or numerical.

Relation to standard techniques. If all variables are scaled on the numerical level, categorical principal components analysis is equivalent to standard principal components analysis.

More generally, categorical principal components analysis is an alternative to computing the correlations between non-numerical scales and analyzing them using a standard principal components or factor-analysis approach. Naive use of the usual Pearson correlation coefficient as a measure of association for ordinal data can lead to nontrivial bias in estimation of the correlations.