Lecture 9: Research Questions for Group Differences IV (Interaction for between-subjects)

Question 1

What is an interaction effect?

Answer 1

Differences due to the combined effects of both factors A and B, over and above that of their individual direct effects (e.g diagnosis x sex)

Question 2

What are the two approaches to analysing the interaction effects?

Answer 2

1. Dummy variable approach (factors substituted with 0 and 1, and then analyse ANOVA results for interaction)
2. Linear contrast approach

Question 3

What are main effect contrasts?

Answer 3

They compare the cell means of the two-way table to investigate contrasts of each factor in the two design.

Question 4

What are interaction contrasts?

Answer 4

They compare the cell means of the two-way table using the cross-product of contrast weights from the linear comparisons for the main effect linear contrasts.

Question 5

What is the cell means approach?

Answer 5

Analysing the interaction effect, by subtracting the marginal effects for the IVs and the grand mean from the observed cell means.

Question 6

What does order-0 and order-1 refer to?

Answer 6

Order-0 refers to a scaling for a difference between two means (main effects), and order-1 refers to a scaling for the difference between two sets of differences between means (interaction effects).