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7.15.1.3 Orthogonal Contrasts
WenoteinTable7.3whetherornotthecontrastisrestrictedtoasetof
orthogonal comparisons. As can be seen from the table, only Difference
and Helmert contrasts are orthogonal; the remaining contrasts are not
orthogonal.
7.15.1.4 Reference Group
Some of these preset contrasts are structured such that a reference group or
category is either defined by the procedure or requires the user to identify
the one that will be used as such. In specifying the reference category,
SPSS recognizes groups according to the values given to the levels of the
independent variable. To illustrate this using our SAT study time example,
recall that there are five levels of the study time independent variable. For
SPSS, the lowest value code (0 in our example) identifies the first level or
group, the next lowest value code (2 in our example) identifies the second
level or group, and so on.
For some of these contrasts where the reference group is not defined
by the procedure, SPSS provides a way for users to specify the reference
group. Using syntax, any group can be specified by users as the reference.
In point-and-click mode, users are presented with only two choices for
the reference group: the first group or the last group. Clicking the button
corresponding to the relevant group specifies the reference group for the
analysis.
7.15.2 DESCRIPTION OF THE CONTRASTS
7.15.2.1 Deviation Contrasts
The reference group (which may be either the first or last group) is not
considered in isolation in the analysis. Each remaining group is compared
to the grand mean of all of the groups (which includes the reference
group).
7.15.2.2 Difference Contrasts
Each group except for the first group is compared to the average of the
groups coded with lower values (the first group by definition has no
groups with value codes below it). In our SAT study time example, the
eight-month study group would be compared to the average of the groups
having studied for zero, two, four, and six months; the six-month study
group would be compared to the average of the groups having studied
for zero, two, and four months; and so on. This is the mirror image of
Helmert contrasts.
7.15.2.3 Dunnett Contrasts
The reference group (first or last) is the focus of the analysis, and is (arbi-
trarily) labeled in the dialog window as the Control group. It is separately
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