Lecture 13Psyc 300A
Review• Confounding, extraneous variables • Operational definitions • Random sampling vs random
assignment• Internal validity• Null hypothesis • Type I and type II errors
Review: Confounding and extraneous variables
• Extraneous variables can be confounds, but can also add variability (noise). For each, provide extraneous variable and confound:
• Study 1: Effect of distraction on pain perception using cold immersion.
• Study 2: Do girls benefit from sixth grade middle school?
Review: Operational definitions• For each of the previous studies,
operationalize the IV and DV
Review: Random sampling vs random assignment
• What is the difference between the two?
• Random assignment is a way to prevent confounding
Review: Internal validity• What is internal validity?• Internal validity: Ability to make
valid inferences concerning the relationship between the IV and DV in an experiment. (effect on the DV is caused only by the IV)
Type I and Type II ErrorsAccept the Null Hypothesis
Reject the Null Hypothesis
Null is really True(chance is responsible)
Correct Decision
Type I Error
Null is really False(chance is not responsible)
Type II Error
Correct Decision
Power• Power is the probability of avoiding
a Type II error.• Power is related to:
– Alpha level– Effect size (mean and sd)– Number of participants
Using More Than Two Levels of an IV• What is a level of an IV?
• In an experiment with an experimental and control group, how many levels?
• Can we have more than two levels?• Example:
– Golf club study– Anxiety management techniques for
speech-giving• Graphing the relationship
Advantages of Multi-level Designs• Efficiency (fewer participants
needed and less time)• Ability to see relationships better
– Ex: Caffeine and Performance (0, 2, 4 cups of coffee)
Graphing Relationships: One IV
0
1
2
3
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5
6
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8
9
10
Romantic Action
Movie type
Ratin
g
0
2
4
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Romantic Action
Movie TypeRating
Multifactor Designs• Factorial design: A design in which
all levels of each IV are combined with all levels of the other IVs.
• Advantages of factorial designs:– More efficient (fewer participants and
less experimenter time)– Allows us to see how variables
interact
Example: Movie Preferences
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Romantic Action
Movie Type
Rat
ing
menwomen
Men Women Mean
Romantic
3 6 4.5
Action 7 4 5.5
Mean 5 5
What a Factorial Design Tells You• Main effect: The effect of an IV on
the DV, ignoring all other factors in the study
• Interaction effect: When the effect of one IV on a DV differs depending on the level of a second IV.
• Graphing a factorial design• Interpreting the interaction first
Examples of Main Effects and Interactions
• A1= morning• A2= late
afternoon
• B1= high fat diet• B2= low fat diet
• DV: 0-50 rating of energy level
More Main Effects and Interactions• A1= morning
• A2= late afternoon
• B1= high fat diet• B2= low fat diet
• DV: 0-50 rating of energy level
More Main Effects and Interactions• A1= morning
• A2= late afternoon
• B1= high fat diet• B2= low fat diet
• DV: 0-50 rating of energy level
Group Activity: Main Effects and InteractionsMake graphs of the
following situations:
Var A Var B AxB interaction
p < .05 n.s. p < .05
p < .05 p < .05 p < .05
n.s. p < .05 n.s.
n.s. n.s. p < .05
Factorial Designs: Naming Conventions
• The first number is the number of levels in first IV, second number is number of levels in second IV, etc.
• 2 x 2• 2 x 3• 2 x 2 x 3• Between-subjects,
repeated measures (within), mixed
A 2 x 3 Interaction
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Relaxation Focus Affirmation
Coping Technique
Rat
ing hi stress
lo stress