SPSS® 12.0 Command Syntax Reference

806 LOGISTIC REGRESSION
CATEGORICAL Subcommand
CATEGORICAL identifies independent variables that are nominal or ordinal. Variables that
are declared to be categorical are automatically transformed to a set of contrast variables
as specified on the CONTRAST subcommand. If a variable coded as 0 – 1 is declared as
categorical, its coding scheme is given indicator contrasts by default.
• Independent variables not specified on CATEGORICAL are assumed to be at least interval
level, except for string variables.
• Any variable specified on CATEGORICAL is ignored if it does not appear either after WITH
on the VARIABLES subcommand or on any METHOD subcommand.
• Variables specified on CATEGORICAL are replaced by sets of contrast variables. If the
categorical variable has n distinct values, there will be n – 1 contrast variables generated.
The set of contrast variables associated with a categorical variable is entered or removed
from the model as a step.
• If any one of the variables in an interaction term is specified on CATEGORICAL, the interaction
term is replaced by contrast variables.
• All string variables are categorical. Only the first eight characters of each value of a string
variable are used in distinguishing between values. Thus, if two values of a string variable
are identical for the first eight characters, the values are treated as though they were the same.
Example
LOGISTIC REGRESSION PASS WITH GPA, GRE, MAT, CLASS, TEACHER
/CATEGORICAL = CLASS,TEACHER.
• The dichotomous dependent variable PASS is regressed on the interval-level independent
variables GPA, GRE, and MAT and the categorical variables CLASS and TEACHER.
CONTRAST Subcommand
CONTRAST specifies the type of contrast used for categorical independent variables. The interpretation
of the regression coefficients for categorical variables depends on the contrasts
used. The default is INDICATOR. The categorical independent variable is specified in parentheses
following CONTRAST. The closing parenthesis is followed by one of the contrast-type
keywords.
• If the categorical variable has n values, there will be n –
1 rows in the contrast matrix.
Each contrast matrix is treated as a set of independent variables in the analysis.
• Only one categorical independent variable can be specified per CONTRAST subcommand,
but multiple CONTRAST subcommands can be specified.
The following contrast types are available. See Finn (1974) and Kirk (1982) for further
information on a specific type.
INDICATOR(refcat) Indicator variables. Contrasts indicate the presence or absence of
category membership. By default, refcat is the last category (represented
in the contrast matrix as a row of zeros). To omit a category
other than the last, specify the sequence number of the omitted category
(which is not necessarily the same as its value) in parentheses
after the keyword INDICATOR.