Review Interactions

Lecture outline 12.1 focuses on understanding interactions. Going over tables 12.4 (as well as tables 12.3, 12.5, Handout 12.2) and this worksheet will expose students a variety of analogies and examples that will help students grasp the important, but hard to understand, concept of interaction.

You might supplement these examples with examples from recent research, such as the finding that the use of artificial bright lights for treating winter depression works well for patients that sleep a lot, have increased appetite, carbohydrate cravings, and who tend to feel worst in the evening, but not as well for patients who have insomnia, anxiety, suicidal tendencies, and who feel worst in the morning.

To help students understand interactions, you might want to discuss interactions involving drugs (such examples also emphasize the importance of interactions--it can be a matter of life or death). This link can help you find  specific examples of interactions involving prescription drugs.. Once students understand interactions at the conceptual level, they will still need some practice before they are able to recognize and interpret interactions in data. Specifically, they will have trouble in two areas:

1. interpreting tables of means; and
2. confusing main effects with interactions when interpreting ANOVA summary tables.
To give students the necessary practice with tables of means, go over tables 12.5 - 12.11. (If you haven't already, have them go through the advanced factorial design worksheet). If students still have trouble, have them graph the table's results (this may take some time). Then, show them that with interactions, the lines are not parallel. They either cross or, if they were extended, they would cross. Students rapidly grasp this idea. However, make sure that they understand that there is more to interactions than just lines crossing. Reiterate the points made in tables 12.2 - 12.3.

An excellent article that explains the concept of interactions extremely well is

Astin, A. W. (1993). An empirical typology of college students. Journal of College Student

Development, 34, 36-44.
We often have students get in groups of 2-3 and read discuss the first two pages of this article.

For a more detailed presentation of discussing interactions, see

Schaefer, V. H. (1988). Teaching the concept of interaction and sensitizing students to its

implications. In Ware, M. E. & Brewer, C. L. (Eds.), Handbook for teaching

statistics and research methods. Hillsdale, NJ: Lawrence Erlbaum Associates.

Back to Chapter 12 Main Menu