Interaction Effects, Factorial Designs, and Their Analysis in Psychology

What is an interaction effect?

Whether the effect of the original independent variable depends on the level of another independent variable.

How does an interaction effect relate to moderation?

The moderator is the other independent variable that changes the relationship.

What do experiments with two independent variables examine?

How two factors work independently and together to influence an outcome; tells you if there is a difference in differences.

What is a cross-over interaction?

An effect where one level of the moderator has a significant positive association and another level has a significant negative association; effect is completely different.

What is a spreading interaction?

An effect that is stronger for one group or another, where the effect completely flips; lines are not parallel.

What is a factorial design?

A design with two or more independent variables where all possible outcomes are compared.

What does a 2 x 2 factorial design mean?

Two levels of one independent variable are crossed with two levels of another independent variable.

What is a between-subjects factorial design?

Each participant is in only one condition for each independent variable.

What is a within-subjects factorial design?

Each participant experiences all conditions of all independent variables.

What is a mixed factorial design?

Includes at least one between-subjects independent variable and at least one within-subjects independent variable.

How can factorial designs test the limits of a research finding?

By testing whether an independent variable affects different kinds of people or situations in the same way.

What is a main effect?

The overall effect of one independent variable on the dependent variable, averaging over the levels of the other independent variable.

What are marginal means?

The arithmetic means for each level of an independent variable, averaging over levels of the other independent variable.

When do you interpret main effects?

When calculating the difference between marginal means and evaluating statistical significance.

What does it mean when the interaction term is statistically significant?

The effect of one independent variable depends on the level of the other independent variable. If so, look at simple effects!

How do researchers describe interactions?

Using phrases like 'It depends on', 'Only under certain conditions', and 'The relationship differs across groups/levels'.

Why are interactions considered more important than main effects?

They reveal the combined effect of two independent variables, showing the real pattern of what's happening.

How does sample size relate to factorial designs?

Between-subjects requires more participants, while within-subjects requires fewer.

What does a 2x4 factorial design mean?

IV1 has 2 levels, IV2 has 4 levels; resulting in 8 conditions.

What does a 3x4 factorial design mean?

IV1 has 3 levels, IV2 has 4 levels; resulting in 12 conditions.

What does a 2x2x2 factorial design mean?

Two levels for each of three independent variables; resulting in 8 conditions.

How would you spot a 2x2x2 (three-way interaction) in research?

The difference in differences is different, indicating an interaction between two IVs depending on a third variable.

What does a three-way interaction show?

A two-way interaction exists for one level of a third IV but not for another.