SPSS® 12.0 Command Syntax Reference

COXREG 299
• 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 variable can be specified per CONTRAST subcommand, but multiple
CONTRAST subcommands can be specified.
• You can specify one of the contrast keywords in the parentheses after the variable specification
to request a specific contrast type.
The following contrast types are available:
DEVIATION(refcat) Deviations from the overall effect. This is the default. The effect for
each category of the independent variable except one is compared to
the overall effect. Refcat is the category for which parameter estimates
are not displayed (they must be calculated from the others). By default,
refcat is the last category. 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 DEVIATION.
SIMPLE(refcat) Each category of the independent variable except the last is compared
to the last category. To use a category other than the last as the omitted
reference category, specify its sequence number (which is not
necessarily the same as its value) in parentheses following the keyword
SIMPLE.
DIFFERENCE Difference or reverse Helmert contrasts. The effects for each category
of the covariate except the first are compared to the mean effect of the
previous categories.
HELMERT Helmert contrasts. The effects for each category of the independent
variable except the last are compared to the mean effects of subsequent
categories.
POLYNOMIAL(metric) Polynomial contrasts. The first degree of freedom contains the linear effect
across the categories of the independent variable, the second contains the
quadratic effect, and so on. By default, the categories are assumed to be
equally spaced; unequal spacing can be specified by entering a metric
consisting of one integer for each category of the independent variable in
parentheses after the keyword POLYNOMIAL. For example, CONTRAST
(STIMULUS) = POLYNOMIAL(1,2,4) indicates that the three levels
of STIMULUS are actually in the proportion 1:2:4. The default metric is
always (1,2,...,k), where k categories are involved. Only the relative
differences between the terms of the metric matter: (1,2,4) is the same metric
as (2,3,5) or (20,30,50) because, in each instance, the difference between the
second and third numbers is twice the difference between the first and second.
REPEATED Comparison of adjacent categories. Each category of the independent
variable except the first is compared to the previous category.
SPECIAL(matrix) A user-defined contrast. After this keyword, a matrix is entered in
parentheses with k –
1 rows and k columns, where k is the number of
categories of the independent variable. The rows of the contrast matrix
contain the special contrasts indicating the desired comparisons
between categories. If the special contrasts are linear combinations of
each other, COXREG reports the linear dependency and stops