Experimental Designs
Experiments are conducted to
identify how independent
variables influence some
change in a dependent
variable.
Researcher-Related Threats to
Internal Validity
Experimenter Effect
 Observer Bias
 Researcher Attribute Effect

Participant-Related Threats to
Internal Validity
The Hawthorne Effect
 Testing Effect
 Maturation
 Experimental Mortality
 Selection Biases
 Intersubject Biases
 Compensatory Study
 Demoralization

Procedure-Related Threats to
Internal Validity
History
 Instrumentation
 Treatment Confound
 Statistical Regression
 Compensation

Exercising Control
Creating Equivalent Groups (Treatment &
Control)
 Manipulating an Independent Variable
 Controlling for extraneous variables

Types of Experimental Designs
Pre-Experimental Design
 Quasi-Experimental Design
 “True” or Classical Experimental Design

Pre-Experiments
 Little
control exercised by researcher
 Conditions are not randomly assigned
 Independent variable is either
manipulated or observed
 Types of Designs:
– One-Shot Case Study
– One-Group Pretest-Posttest Design
– Static-Group Comparison
Quasi-Experiments
Some Control exerted by researcher
 Groups not randomly assigned -- assigned
by pretest or natural categories – called
“matching”
 Independent variable is often observed in
its naturally occurring context
 Tend to be field experiments
 Types of Designs:

– Time-Series Designs
– Nonequivalent Control Group Design
– Multiple Time-Series Design
True or Classical Experiments
Most controlled design
Must have random assignment to groups
Laboratory experiment
Independent variable is manipulated
Double-Blind Experiment is when the
participants and those who have contact with
the participants are unaware of the group to
which a participant is assigned.
 Manipulation checks are used to ensure the
operationalization of the independent variable
was manipulated as intended.


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Types of Classical Experiments
Pretest – Posttest Control Group Design
 Posttest – Only Control Group Design
 Solomon Four – Group Design

Factorial Designs
 Used
when there is more than one
independent variable
 Examines complex causal
relationships
– how each independent variable affects
the dependent variable (main effects)
and how the independent variables
combined affect the dependent variable
(interaction effects)
RQ: How does the sex of the speaker and
immediacy influence perceived credibility?
High
Immediacy
Female
Speaker
Male
Speaker
Moderate
Immediacy
Low
Immediacy
Female
Speaker/High
Immediacy
Female
Speaker/ Mod
Immediacy
Female
Speaker/Low
Immediacy
Male
Speaker/High
Immediacy
Male
Speaker/Mod
Immediacy
Male
Speaker/Low
Immediacy
2 x 3 Factorial Design