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School

University of Guelph
Department

Family Relations and Human Development

Course Code

FRHD 3070

Professor

Hendry

Results of Factorial Designs

Main Effects

Determining if an independent variable has had an effect on the dependent variable while ignoring the

other independent variables in the design

Main effect - separate effects of each independent variable regardless of the other independent

variable

For each independent variable, you may have a main effect

E.g., A 2 X 2 factorial designs yields 2 main effects, one for each factor

A main effect is an outcome that is a consistent difference between levels of an IV.

The row mean differences depict the main effect for one factor

For iî…¶staî…¶ce, we would say thereâ€™s a î…µaiî…¶ effect for coî…¶teî…¶t if we fiî…¶d a statistical difference

between the averages for the <1, <5, and >5 groups at both levels of pacing.

The column mean differences depict the main effect for the second factor

Alterî…¶ately, we would say thereâ€™s a î…µaiî…¶ effect for duratioî…¶ if we fiî…¶d a statistical difference

between the averages for the different pacing groups at all levels of duration.

Interactions

Occur when the effect of one independent variable depends on the levels of another independent

variable

An interaction represents the joint effect of the IVs on the DV.

An interaction effect is depicted in a graph by the presence of nonparallel data lines, or lines that cross

or appear to cross at some time in the future.

How do you detect interactions?

1. Statistical analysis will report on all main effects and interactions.

2. When can't talk about effect on one factor without mentioning the other factor. When you

have an interaction it is impossible to describe your results accurately without mentioning both

factors.

3. You can identify an interaction in the graphs of group means: whenever there are lines that are

not parallel there is an interaction present.

Note that an interaction effect can occur when:

There are no main effect(s) for any independent variable (as in the above example)

There is a main effect for independent variable A but not B;

There is a main effect for independent variable B but not A

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