overview of experimental research design
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Experimental Research Design
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Overview
Topics Appropriate to Experiments
The Classical Experiment
Selecting Subjects
Variations on Experimental Designs
An Illustration of Experimentation
"Natural" Experiments
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Topics Appropriate to
Experiments
Projects with limited and well-defined concepts.
Projects that are explanatory rather than descriptive.
Studies of small group interaction.
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Components of
Experiments
Three Pairs
Independent and dependent variables
Pretesting and posttesting
Experimental and control groups
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Broad DistinctionsBetween subjects
Dependent measures taken one time
Data are independent (i.e., not correlated)
Within subjects
A “repeated measures” design
Dependent measures taken multiple times
Data are dependent
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Design Types
Single factor (one-way)
Studies one independent variable
Multi-factor Studies multiple independent variables
May have several levels
Examples:
Two-way (e.g., 2 x 2)
Three-way (e.g., 2 x 2 x 2)
Time-series
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Experimental and
Control Groups
Must be as similar as possible.
Control group represents what the experimental
group would have been like had it not been exposed
to the experimental stimulus.
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Selecting Subjects/Cases
Probability sampling
Randomization
Matching
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Randomization and
MatchingMay not know which variables will be relevant for
matching process.
Most statistics used to analyze results assume
randomization.
Randomization only makes sense if you have a largepool of subjects.
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Preexperimental
Research DesignsOne-shot case study - single group of subjects is
measured on a variable following experimental
stimulus.
One-group pretest-posttest design - adds a pre-test
for the group, but lacks a control group.
Static-group comparison - includes experimental
and control group, but no pre-test.
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Sources of Internal
InvalidityHistorical events may occur during the course of the
experiment.
Maturation of the subjects.
Testing and retesting can influence behavior.
Instrumentation
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Sources of Internal
InvalidityStatistical regression of subjects starting out in
extreme positions.
Selection biases.
Experimental mortality - subjects drop out of the
study before it's completed.
Demoralized control group subjects.
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Limiting External
Invalidity
Solomon four-group design
Posttest-only control group design
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Solomon Four-group
DesignFour groups of subjects, assigned randomly:
Groups 1 and 2 are the control and experimental
group.
Group 3 does not have the pre-test.
Group 4 is only post tested (no stimulus; i.e. IV)
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Solomon 4-group design
A di"erent design can allow us to look at the
e"ects of a pretest
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R X O
R OR O X O
R O O
If these two groups are different, pretestsensitization is an issue.
Pre X Treatment interaction
If these two groups are different, there is atesting effect in general.
Solomon 4-group designIncluding a pretest can sensitize participants and
create a threat to construct validity. Combining
the two basic designs creates the Solomon 4-
group design, which can determine if pretest
sensitization is a problem:
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Posttest-only Control
Group Design
Includes Groups 3 and 4 of the Solomon design.
With proper randomization, only these groups are
needed to control the problems of internal invalidity
and the interaction between testing and stimulus.
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"Natural" Experiments
Important social scientific experiments occur outsidecontrolled settings and in the course of normal social
events.
Raise validity issues because researcher must take
things as they occur.
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Experimental Method
Strengths:
Isolation of the experimental variable over time.
Experiments can be replicated several times using
di"erent groups of subjects.
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Experimental Method
Weaknesses:
Artificiality of laboratory setting.
Social processes that occur in a lab might not occur
in a more natural social setting.