Experiments

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Experiments
Pierre-Auguste Renoir Barges on the Seine, 1869
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• Topics Appropriate to Experiments
• 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.
• Laboratory and Field Experiments
• Field experiments provide a natural setting, but
  allow for less control over variables.

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• The Classical Experiment
• 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.

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• The Classical Experiment
• 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

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• The Classical Experiment
• 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.
• 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).

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The Classical Experiment
Experimental Control Schedule Group Group Time
1 Measure Dependent Measure Dependent Variable
(pretest) Variable (pretest) Time 2 Administer
Stimulus Time 3 Measure Dependent Measure
Dependent Variable (posttest) Variable (posttest)
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• The Classical Experiment
• 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.)

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• The Classical Experiment
• 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 subjects
  outcomes more favorably.
• In the double-blind experiment, neither the
  subject nor the experimenter know which subjects
  are in the experimental group.

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• Selecting Subjects
• 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.

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• Selecting Subjects
• Probability Sampling
• Probability sampling is used to achieve
  representativeness with large samples.
  Therefore, it is not often used for experiments.
• Randomization
• It is essential for subjects to be randomly
  assigned to the experimental and control groups.
• Matching
• To assure even distribution of key
  characteristics between groups, experimenters
  might assign subjects to groups.

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• Quasi-Experimental Designs
• 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.

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• Quasi-Experimental Designs
• One-Shot Case Study
• Posttest only of the experimental group.

Schedule Experimental Group Control Group Time
1 Time 2 Administer Stimulus Time 3 Measure
Dependent Variable (posttest)
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• Quasi-Experimental Designs
• One-Group Pretest-Posttest Design
• Pretest and posttest of one group.

Schedule Experimental Group Control Group Time
1 Measure Dependent Variable (pretest) Time
2 Administer Stimulus Time 3 Measure
Dependent Variable (posttest)
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• Quasi-Experimental Designs
• Static Group Comparison
• Control at Time 3.

Schedule Experimental Group Control Group Time
1 Time 2 Administer Stimulus Time 3 Measure
Dependent Measure Dependent Variable
(posttest) Variable (posttest)
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Experimental Designs This chart summarizes the
experimental designs
Control Group
Yes
No Yes Classical One-group, Pretest-Pos
ttest Pretest No Static-Group One-Shot Compa
rison Case Study
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• Validity Issues in Experiments
• Sources of Internal Invalidity
• Do the results reflect the effect of the stimulus
  variable?
• Factors affecting internal validity
• History Unplanned events that occur during the
  experiment.
• Maturation Change in people from Time 1 to Time
  3.
• Testing (cueing) The process of the experiment
  itself creates changes in the dependent variable.

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• Validity Issues in Experiments
• Sources of Internal Invalidity (Continued)
• Factors affecting internal validity
• Instrumentation Do the pretest and posttest
  measures exactly match each other?
• Regression Toward the Mean Changes might occur
  because subjects begin at the extreme.
• Selection Bias Subjects are not matched across
  groups.

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• Validity Issues in Experiments
• Sources of Internal Invalidity (Continued)
• Factors affecting internal validity
• Experimental Mortality Subjects drop out of the
  study before it is completed.
• Causal Time Order In some cases, it is difficult
  to time the stimulus after the pretest.
• Diffusion Subjects across groups share
  information with one another.
• Compensation Experimenters might treat the
  control group differently.

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• Validity Issues in Experiments
• Sources of Internal Invalidity (Continued)
• Factors affecting internal validity
• Compensatory Rivalry Subjects who know they are
  in the control group might behave with more
  interest.
• Demoralization Subjects in the control group
  might behave with less interest.

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• Validity Issues in Experiments
• 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.

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• Alternative Experimental Settings
• 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

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• Alternative Experimental Settings
• 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.

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Questions?