Experiments
Pierre-Auguste Renoir: Barges on the Seine, 1869
Topics Appropriate to Experiments
1. Experiments Allow for Control of Variables
• How much is learned about a topic.
• How much time is allowed for tasks.
• The composition of groups.
• Who speaks and how long they speak in
groups.
• Manipulation of opinions by the use of
confederates in group settings.
2. Laboratory and Field Experiments
• Field experiments provide a natural setting, but
allow for less control over variables.
The Classical Experiment
1. Independent and Dependent Variables
• Typically, the operational definitions of
independent and dependent variables are
defined prior to the experiment.
• The way in which independent variables will be
introduced into the experiment is
predetermined.
• The purpose of an experiment is to manipulate
the score on the dependent variable by
introducing different independent variables or
manipulating existing ones.
The Classical Experiment
1. Independent and Dependent Variables (Cont.)
• Included among the independent variables are
one or more variables specifically intended to
manipulate the score on the dependent
variable.
• These are called by various names, each
meaning the same thing:
• Stimulus
• Treatment
• Experimental
The Classical Experiment
2. Pretesting and Posttesting
• The pretest is the measurement of variables
prior to introducing the treatment variable.
• The posttest is the measurement of variables
after introducing the treatment variable.
3. Experimental and Control Groups
• The Experimental group is exposed to the
treatment variable(s).
• The Control group is not exposed to the
treatment variable(s).
The Classical Experiment
Experimental
Group
Control
Group
Time 1
Measure Dependent
Variable (pretest)
Measure Dependent
Variable (pretest)
Time 2
Administer Stimulus
Time 3
Measure Dependent
Variable (posttest)
Schedule
Measure Dependent
Variable (posttest)
The Classical Experiment
4. The Blind Experiment
• In some cases, the experimenter might
influence the scores on the variables.
• Example: In evaluating the efficacy of a new
medicine, if the subjects know they are taking
the medicine, they might respond because of
this knowledge rather than because of the
medicine. Therefore, all subjects are given
“medicine,” but the control group is given a
placebo: a false medicine (e.g., a pill filled with
sugar rather than medicine.)
The Classical Experiment
5. The Double Blind Experiment
• In some circumstances, the experimenter
might influence scores on the variables.
• Example: If the experimenter knows which
subjects are taking the real medicine and this
person wants the medicine to be effective, then
the experimenter might evaluate the subject’s
outcomes more favorably.
• In the double-blind experiment, neither the
subject nor the experimenter know which
subjects are in the experimental group.
Selecting Subjects
1. Representation
• Typically, experiments focus on building or
testing theory rather than attempting to predict
population characteristics.
• Therefore, as long as the subjects have key
characteristics of interest, then it is not often
necessary that the sample be representative.
• It is critical, however, for subjects to be evenly
matched in characteristics across the
experimental and control groups.
Selecting Subjects
2. Probability Sampling
• Probability sampling is used to achieve
representativeness with large samples.
Therefore, it is not often used for experiments.
3. Randomization
• It is essential for subjects to be randomly
assigned to the experimental and control
groups.
4. Matching
• To assure even distribution of key
characteristics between groups, experimenters
might assign subjects to groups.
Quasi-Experimental Designs
1. Rationale
• Sometimes, the researcher examines events in
the field that cannot be easily anticipated.
• Responses to disasters.
• Responses to rapid social change.
• Sometimes, the added expense of a classical
experiment is not necessary.
• The one-shot case study is common to
market testing of low-involvement products.
Quasi-Experimental Designs
2. One-Shot Case Study
• Posttest only of the experimental group.
Schedule
Experimental Group
Time 1
Time 2
Administer Stimulus
Time 3
Measure Dependent
Variable (posttest)
Control Group
Quasi-Experimental Designs
3. One-Group Pretest-Posttest Design
• Pretest and posttest of one group.
Schedule
Experimental Group
Time 1
Measure Dependent
Variable (pretest)
Time 2
Administer Stimulus
Time 3
Measure Dependent
Variable (posttest)
Control Group
Quasi-Experimental Designs
4. Static Group Comparison
• Control at Time 3.
Schedule
Experimental Group
Control Group
Time 1
Time 2
Administer Stimulus
Time 3
Measure Dependent
Variable (posttest)
Measure Dependent
Variable (posttest)
Experimental Designs
This chart summarizes the experimental designs:
Control Group
Yes
No
Yes
Classical
One-group,
Pretest-Posttest
No
Static-Group
Comparison
One-Shot
Case Study
Pretest
Validity Issues in Experiments
1. Sources of Internal Invalidity
• Do the results reflect the effect of the stimulus
variable?
• Factors affecting internal validity:
1. History: Unplanned events that occur
during the experiment.
2. Maturation: Change in people from Time 1
to Time 3.
3. Testing (cueing): The process of the
experiment itself creates changes in the
dependent variable.
Validity Issues in Experiments
1. Sources of Internal Invalidity (Continued)
• Factors affecting internal validity:
4. Instrumentation: Do the pretest and
posttest measures exactly match each
other?
5. Regression Toward the Mean: Changes
might occur because subjects begin at the
extreme.
6. Selection Bias: Subjects are not matched
across groups.
Validity Issues in Experiments
1. Sources of Internal Invalidity (Continued)
• Factors affecting internal validity:
7. Experimental Mortality: Subjects drop out
of the study before it is completed.
8. Causal Time Order: In some cases, it is
difficult to time the stimulus after the
pretest.
9. Diffusion: Subjects across groups share
information with one another.
10. Compensation: Experimenters might treat
the control group differently.
Validity Issues in Experiments
1. Sources of Internal Invalidity (Continued)
• Factors affecting internal validity:
11. Compensatory Rivalry: Subjects who know
they are in the control group might behave
with more interest.
12. Demoralization: Subjects in the control
group might behave with less interest.
Validity Issues in Experiments
2. Sources of External Invalidity
• Can the results be generalized to the
population?
• Interaction: Subjects who know they are being
studied might be more receptive to the
stimulus.
• Cueing: The administration of the pretest might
sensitize subjects to the content of the
stimulus.
Alternative Experimental Settings
1. Web-Based Experiments
• Subjects answer questions or perform tasks
online.
• Subjects might be asked questions prior to
being assigned to a group, or they might be
assigned at random at the outset of their
session.
See: Online Social Psychology Studies
See: Small World Phenomenon
Alternative Experimental Settings
2. Natural Experiments
• Behavior occurring during or after natural
events can be investigated.
• “Control” groups can be persons in similar
settings that did not experience the natural
event.