PSY 429 Experimental Psychology

Class Exercise #8

 

Main Effects and Interactions in a Two-Way Independent-Measures Analysis of Variance

Answers

 

1. What is a main effect?

 A main effect is the sole effect of one IV on the DV in an analysis of variance with more than one IV.

2. What is an interaction effect?

 An interaction is the effect of the levels of one IV on the DV as a function of the levels of the other IV. If one IV significantly affects the DV depending on scores on the other IV, than an interaction is present.

3. What are the three components of SS-treatment in a two-way analysis of variance?

 1) main effect of Factor A (IV₁)

2) main effect of Factor B (IV₂)

3) the interaction between Factor A and Factor B

4. Sketch a graph for each of the following sets of data. Assume Factor A is the number of words in a word list which participants must learn, either 40 words or 20 words in the list. Assume Factor B is the type of words used in the word list, either concrete words (words in which a physical referent can be imaged, e.g. book, chair, apple, horse) or abstract words (words lacking a physical referent, e.g. include, describe, intend, complex).

State whether or not the graph indicates the presence of an interaction between Factors A and B and be complete in your statement of what the graph indicates.

a.

Recall of the words was greatest for concrete words, regardless of how many words were in the list. Recall was also greatest for the shorter list regardless of whether the words were concrete or abstract. There is no significant interaction between the number of words in the list and whether the word is an abstract or concrete word.

b.

A significant interaction was observed. Recall was the same for the list containing 40 words regardless of whether the words were concrete or abstract. On the other hand, for the 20-word list, recall was superior for abstract words as compared to concrete words.

c.

No significant interaction was observed. There was no difference in recall between concrete and abstract words regardless of the length of the list. However, recall was better for the 40-word list than the 20-word list.

5. The data below are the result of a two-factor experiment comparing the weight loss achieved from two different diet programs (Factor A) for men versus women (Factor B).

a. Do the main effects appear significant to you (you do not have to perform an ANOVA) for diet or for gender?

There appears to be a main effect of diet but not of gender.

b. Sketch a graph to determine whether an interaction between diet and gender is likely.

c. As a result of what you discovered in doing a and b above, what would you state as your conclusion?

 No difference was found between men and women. They lost approximately the same amoutn of weight. However, diet 1 appeared to be more effective than diet 2 for both men and women. No significant interaction was observed.

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