SPSS® 12.0 Command Syntax Reference

298 COXREG
STRATA Subcommand
STRATA identifies a stratification variable. A different baseline survival function is computed
for each stratum.
• The only specification is the subcommand keyword with one, and only one, variable name.
• If you have more than one stratification variable, create a new variable that corresponds
to the combination of categories of the individual variables before invoking the COXREG
command.
• There is no limit to the number of levels for the strata variable.
Example
COXREG SURVIVAL WITH GROUP
/STATUS SURVSTA (1)
/STRATA=LOCATION.
• STRATA specifies LOCATION as the strata variable.
• Different baseline survival functions are computed for each value of LOCATION.
CATEGORICAL Subcommand
CATEGORICAL identifies covariates that are nominal or ordinal. Variables that are declared to
be categorical are automatically transformed to a set of contrast variables (see “CONTRAST
Subcommand” below). If a variable coded as 0 – 1 is declared as categorical, by default, its
coding scheme will be changed to deviation contrasts.
• Covariates not specified on CATEGORICAL are assumed to be at least interval, except for
strings.
• Variables specified on CATEGORICAL but not on VARIABLES or any METHOD subcommand
are ignored.
• 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 one categorical variable are entered
or removed from the model together.
• 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 among 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.
CONTRAST Subcommand
CONTRAST specifies the type of contrast used for categorical covariates. The interpretation
of the regression coefficients for categorical covariates depends on the contrasts used. The
default is DEVIATION. For illustration of contrast types, see the appendix.
• The categorical covariate is specified in parentheses following CONTRAST.