Statistics for the Behavioral Sciences by Frederick J. Gravetter, Larry B. Wallnau ISBN 10: 1305504917 ISBN 13: 9781305504912

466 CHAPTER 14 | Two-Factor Analysis of Variance (Independent Measures)
FIGURE 14.4
Sample means for the data in
Example 14.2. The data are quiz
scores from a two-factor study
examining the effect of studying
text on paper vs. on a computer
screen for either a fixed time or a
self-regulated time.
Mean
quiz
score
10
9
8
7
6
5
4
3
2
1
Paper
Computer
screen
Mode of Text Presentation
Fixed time
Self-regulated time
■ Interpreting the Results from a Two-Factor ANOVA
Because the two-factor ANOVA involves three separate tests, you must consider the overall
pattern of results rather than focusing on the individual main effects or the interaction. In
particular, whenever there is a significant interaction, you should be cautious about accepting
the main effects at face value (whether they are significant or not). Remember, an
interaction means that the effect of one factor depends on the level of the second factor.
Because the effect changes from one level to the next, there is no consistent “main effect.”
Figure 14.4 shows the sample means obtained from the paper vs. computer screen study.
Recall that the analysis showed that both main effects were not significant but the interaction
was significant. Although both main effects were too small to be significant, it would
be incorrect to conclude that neither factor influenced behavior. For this example, the difference
between studying text presented on paper vs. on a computer screen depends on how
studying time is controlled. Specifically, there is little or no difference between paper and a
computer screen when the time spent studying is fixed by the researchers. However, studying
text from paper produces much higher quiz scores when participants regulate their own
study time. Thus, the difference between studying from paper and studying from a computer
screen depends on how the time spent studying is controlled. This interdependence
between factors is the source of the significant interaction.
LEARNING CHECK
1. What happens in the second stage of a two-factor ANOVA?
a. The total variability is divided into between treatments and within treatments.
b. The variability within treatment is divided into between subjects and error.
c. The variability within treatments is divided into the two main effects and the
interaction.
d. The variability between treatments is divided into the two main effects and the
interaction.