Multiple Comparison Procedures - Analysis of Variance Designs - page 163

Contrast Coefficients

months of study

eight
months Contrast zero months two months four months six months

1
2
3

4
0
0

-1
1
1

-1
-1
1

-1
0
-1

-1
0
-1

Figure 7.25 The Contrast Coefficients table.

your weights. In Figure 7.24, we can see that the total is equal to zero
(note that SPSS will not multiply your coefficients for you as you
enter subsequent contrasts).

At the completion of the first contrast, click the Next pushbutton. This
will cause the window to now display Contrast 2 of 2 and (probably to
your surprise and perhaps to your distress) to blank out the large middle
panel. Do not panic - SPSS will retain your first set of coefficients. In
an unfortunate formatting decision, SPSS shows only one set of contrast
weights at a time. Repeat what you did for the first contrast, then once
again click the Next pushbutton, which will now blank out the second
set of weights and display Contrast 3 of 3 , and type in the third contrast.
Whenyoufinish,click Continue to return to the main One-Way ANOVA
dialog window and click OK to run the analysis.

7.29 OUTPUT FROMUSER-DEFINED CONTRASTS
ANALYSIS IN SPSS

SPSS prints out the coefficients you used for each contrast in a table called
Contrast Coefficients, which we show in Figure 7.25. The weights for
each contrast are shown horizontally in the display but nonetheless are
invaluable reminders about what you requested. This is important because
SPSS simply labels the contrasts as 1 , 2 , and 3 in the analysis proper, which
is shown in Figure 7.26.

These are the
probabilities of the
differences occurring
by chance alone for
the three contrasts.

Contrast Tests

Contr
ast

Value of
Contrast

Sig. (2-tailed)

Std. Error t df

satscore Assume equal variances 1

2
3
1
2
3

− 612.8571

−

61.54474
19.46216
27.52364
49.97959
21.10872
29.04372

− 9.958 30

30
30
12.744
9.816
21.268

.000
.000
.000
.000
.004
.000

78.5714 − 4.037

− 210.0000 − 7.630

Does not assume equal
variances

− 612.8571

−

− 12.262

−

78.5714 3.722

− 210.0000 − 7.230

Figure 7.26. The Contrast tests.