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EXPERIMENTAL DESIGN
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Describe the relationship between a
confounding variable and the internal validity
of an experiment
Describe the posttest-only design and the
pretest-posttest design, including the
advantages and disadvantages of each design
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Contrast and compare independent groups
design with a repeated measures design
Summarize the advantages and disadvantages
of using a repeated measures design
Describe a matched pairs design, including
reasons to use this design
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Confounding Variable
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Another variable that occurs along with the
independent variable
Is an uncontrolled variable
Cannot determine which variable is responsible for
the effect
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Internal Validity
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An experiment must be designed and conducted so
that only the independent variable can be cause of
the results
Internal validity exists when the results of an
experiment can be confidently attributed to the effect of the
independent variable
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Posttest-Only Design
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Must:
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Obtain two equivalent groups of participants
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Care must be taken to eliminate selection differences
Introduce the independent variable
 Measure the effect of the independent variable on the
dependent variable
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Pretest-Posttest Design
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A pretest is given to each group prior to
introduction of the experimental manipulation
Assures that groups are equivalent at the beginning
of the experiment
Can quickly measure changes that occur from the
pretest to the posttest
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Advantages of the pretest-posttest design
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Mortality (dropout factor)
Assess equivalency of groups with small sample
size
Can be used to select participants for the
experiment
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Disadvantages of the pretest-posttest design
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Time consuming and awkward to administer
Sensitizes participants to what is being studied
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Demand characteristics
Reduces external validity
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Independent Groups Design
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Participants participate in only one group
Repeated Measures Design
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Participants are in all conditions
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Advantages and Disadvantages of Repeated Measures
Design
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Advantages
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Fewer participants
Extremely sensitive to statistical differences
Conditions are identical because person is own control group
Disadvantages
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Order effect
Practice effect
 Fatigue effect
 Carryover effect
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Counterbalancing
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Complete counterbalancing
Latin squares
Time Interval Between Treatments
Choosing Between Independent Groups and
Repeated Measures Design
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Goal is to Match People on a Participant
Characteristic
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Matched to either the dependent measure or a
variable that is strongly related to the dependent
variable