more than one independent variable laying out a factorial design a research example choosing a...
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More Than One Independent Variable
Laying Out a Factorial Design
A Research Example
Choosing a Between-Subjects Design
What is a factor?
More Than One Independent Variable
A factor is an independent variable.
What is a factorial design?
More Than One Independent Variable
A factorial design contains more than one independent variable.
The effect of psychotherapy (IV1) and antidepressant drugs (IV2) on depression (DV).
A two factor experiment is the simplest factorial design.
What information can a factorial design provide?
More Than One Independent Variable
A factorial design can provide information about both treatment and interaction effects.
What is a main effect? What determines the number of main effects in an experiment?
More Than One Independent Variable
A main effect is the action of a single IV on the DV.
There can be as many main effects as independent variables.
Provide an example of a main effect in a hypothetical study of exercise and depression.
More Than One Independent Variable
An experimenter studies the effects of exercise intensity (IV1) and duration (IV2) on depression (DV).
If exercise intensity or duration separately reduced depression, these would constitute main effects.
How do we determine whether we have main effects in our experiment?
More Than One Independent Variable
Perform an appropriate statistical test.
In a 2 x 3 x 3 study, how many IVs and treatment conditions are there?
More Than One Independent Variable
There are 3 independent variables and 18 treatment conditions.
Provide an example of a 2 x 3 x 3 study.
More Than One Independent Variable
The independent variables were the perpetrator’s gender (male or female), relationship to the child (parent, step-parent, or parent’s partner), and severity of the abuse (neurological damage, broken bones, or bruising).
The dependent variable was sentence length.
What is an interaction?
More Than One Independent Variable
An interaction is the joint effect of two or more IVs on the DV.
When there is an interaction, the effect of one IV is different across levels of the other IV.
Provide an example of an interaction.
More Than One Independent Variable
If the antidepressant Paxil produced greater reductions in depression in the Cognitive Behavior Therapy (CBT) condition than the Waiting List condition, this would illustrate an interaction between drug and psychotherapy.
What is a higher-order interaction?
More Than One Independent Variable
A higher-order interaction is an interaction among three or more IVs.
Interpretation can be difficult when more than three IVs interact in an experiment.
Provide an example of a higher-order interaction.
More Than One Independent Variable
A previous hypothetical study examined the effect of a perpetrator’s gender (male or female), relationship to the child (parent, step-parent, or parent’s partner), and severity of the abuse (neurological damage, broken bones, or bruising) on sentencing.
Provide an example of a higher-order interaction.
More Than One Independent Variable
There would be a higher-order interaction if the perpetrator’s gender, relationship to the child, and severity of abuse jointly determined sentence length.
How many interactions are possible in a study with three IVs?
More Than One Independent Variable
Assign letters (A, B, C) to the independent variables. Identify all unique two- and three- treatment combinations.
For three independent variables, these include AB, AC, BC, and ABC. ABC is the higher-orderinteraction.
How does an interaction affect the interpretation of our results?
More Than One Independent Variable
An interaction qualifies a main effect, warning us that there may be limits or exceptions to the effect of an IV on the DV.
When there is an interaction, we must consider both IVs, because the effects of one factor will depend on the levels of the other factor.
Explain the factor-labeling method.
Laying Out a Factorial Design
The factor-labeling method lists the two factors in parentheses after the numerical notation.
For example, 2 x 2 (Type of Name x Length of Name).
Explain the factor and levels method.
Laying Out a Factorial Design
This method lists the two factors and their respective levels after the numerical notation.
For example, 2 x 2 (Type of Name: given, nickname x Length of Name: short, long).
What advantage does the factor and levels method have over the factor-labeling method?
Laying Out a Factorial Design
The factor and levels method provides more detailed information about the design than the factor-labeling method.
Why use a factorial design instead of two separate univariate experiments?
A Research Example
A factorial design is more efficient since it combines several one-factor experiments and allows us to study interactions.
A factorial design can achieve greater external validity since it can better recreate the complexity of the multivariate environment.
Why should we keep between-subjects designs simple?
Choosing a Between-Subjects Design
Practical limitations include:
number of subjects time interpretability of results