User Guide

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Appendix

Categorical Variable

Coding Schemes

In many SPSS procedures, you can request automatic replacement of a categorical

independent variable with a set of contrast variables, which will then be entered or

removed from an equation as a block. You can specify how the set of contrast variables

is to be coded, usually on the

CONTRAST subcommand. This appendix explains and

illustrates how different contrast types requested on

CONTRAST actually work.

Deviation

Deviation from the grand mean. In matrix terms, these contrasts have the form:

where k is the number of categories for the independent variable and the last category

is omitted by default. For example, the deviation contrasts for an independent variable

with three categories are as follows:

mean ( 1/k 1/k ... 1/k 1/k )

df(1) ( 1–1/k –1/k ... –1/k –1/k )

d f ( 2 ) ( – 1 / k 1–1/k ... –1/k –1/k )

df(k–1) ( –1/

k –1/k ... 1–1/k –1/k )

( 1/3 1/3 1/3 )

( 2/3 –1/3 –1/3 )

( –1/3 2/3 –1/3 )