EXPERIMENTS: PART 4
Overview

Review of concepts
Factorial matrix
Main effects
Interactions (possibly hardest concept of the course)

Reminders



 Bring
your CV or resume next time
 Make sure get an APA style manual ASAP
Review

Factor vs. Level
 Single-factor
designs (previous)
 Single-factor,
two-level design
 Single-factor, multi-level design
 Factorial
designs (today)
 Two-factor
design
 Multi-factor design



Factor = categorical independent variable
Level = category within a factor
Condition = experimental group or cell
Factorial Matrix
Factor 1
Level 1
Level 2
Factorial Matrix
Anxiety
Low
High
Factorial Matrix
Anxiety
Low
High
4.3
8.2
What is the effect of anxiety (independent variable)
upon math test performance (dependent variable)?
Factorial Matrix
Factor 1
Level 1 of
Factor 1
Level 2 of
Factor 1
Factorial Matrix
Factor 1
Level 1 of
Factor 1
Level 1 of
Factor 2
Factor 2
Level 2 of
Factor 2
Level 2 of
Factor 1
Factorial Matrix
Factor 1
Level 1 of
Factor 1
Level 2 of
Factor 1
Level 1 of
Factor 2
Condition A
Condition B
Level 2 of
Factor 2
Condition C
Condition D
Factor 2
Factorial Matrix
Anxiety
Low
Absent
Mortality
Salience
Present
High
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.7
4.3
Present
9.2
2.1
Mortality
Salience
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
Medium
High
Absent
5.7
5.6
4.3
Present
9.2
3.8
2.1
Mortality
Salience
Factorial Matrix
What are the effects of mood and mortality
salience on support for cigarette taxes?
Mortality
Salience
Mood
Low
Anxiety
Medium
Anxiety
High
Anxiety
High
Sadness
Absent
5.7
5.6
4.3
5.8
Present
9.2
3.8
2.1
9.1
9.1
3.9
2.0
8.7
(newspaper)
Present
(video)
Notation

Factor = categorical independent variable
Level = category within a factor
Condition = experimental group or cell

Summarized using numbers


2
x3
2 x 4 x 8
 # of Factors = # of numbers
 # of Levels on a factor = numbers themselves
 # of Conditions = product of the numbers
(do the math)
Main Effects

Main effect when a factor (categorical independent
variable) has an effect on the outcome (dependent
variable)
 Significant

mean difference across levels of the factor
Can be as many main effects as there are factors
 One
factor: 0 or 1 main effects
 Two factors: 0, 1, or 2 main effects, etc.
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.7
4.3
5.0
Present
9.2
2.1
5.7
7.5
3.2
Mortality
Salience
How many main effects?
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.7
5.7
5.7
Present
5.7
5.7
5.7
5.7
5.7
Mortality
Salience
How many main effects?
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.7
5.9
5.8
Present
5.7
5.9
5.8
5.7
5.9
Mortality
Salience
How many main effects?
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
7.7
5.9
6.8
Present
7.7
5.9
6.8
7.7
5.9
Mortality
Salience
How many main effects?
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
7.7
7.7
7.7
Present
5.9
5.9
5.9
6.8
6.8
Mortality
Salience
How many main effects?
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.0
3.0
4.0
Present
8.0
4.0
6.0
6.5
3.5
Mortality
Salience
Interactions



Also called “moderators” or “effect modifiers”
Means that the effect of one variable depends on
someone’s status on another variable
Means that the effect of one factor on the outcome
differs depending on where someone falls on
another factor
 One
factor could increase the effect of another factor
 One factor could decrease the effect of another factor
 One factor could change the direction of the effect of
another factor
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
How many main effects?
Anxiety
Low
High
Absent
5.0
7.0
6.0
Present
8.0
6.0
7.0
6.5
6.5
Mortality
Salience
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Does anxiety increase
support for cigarette taxes?
Anxiety
Low
High
Absent
5.0
7.0
6.0
Present
8.0
6.0
7.0
6.5
6.5
Mortality
Salience
Factorial Matrix
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
In absence of mortality salience,
anxiety increases support for taxes
Anxiety
Low
High
Absent
5.0
7.0
6.0
Present
8.0
6.0
7.0
6.5
6.5
Mortality
Salience
Factorial Matrix
In the presence of mortality salience,
anxiety decreases support for taxes
What are the effects of anxiety and mortality
salience on support for cigarette taxes?
Anxiety
Low
High
Absent
5.0
7.0
6.0
Present
8.0
6.0
7.0
6.5
6.5
Mortality
Salience
Video Clip
https://www.youtube.com/watch?v=OE46w0RqmQA
More examples
http://www.psychwiki.com/wiki/What_is_an_Interaction%3F
http://www.unc.edu/courses/2008spring/psyc/270/00
1/interact.html

More graphs next time