1. CHAPTER 8
EXPERIMENTAL DESIGN
© 2012 The McGraw-Hill Companies, Inc.

2. LEARNING OBJECTIVES
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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3. LEARNING OBJECTIVES
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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4. CONFOUNDING AND INTERNAL VALIDITY
Confounding Variable
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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5. CONFOUNDING AND INTERNAL VALIDITY
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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6. BASIC EXPERIMENTS Posttest-Only Design Must:
Obtain two equivalent groups of participants
Care must be taken to eliminate selection differences
Introduce the independent variable
Measure the effect of the independent variable on the dependent variable
© 2012 The McGraw-Hill Companies, Inc.

7. POSTTEST-ONLY DESIGN © 2009 The McGraw-Hill Companies, Inc.

8. BASIC EXPERIMENTS Pretest-Posttest Design
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
© 2012 The McGraw-Hill Companies, Inc.

9. BASIC EXPERIMENTS Advantages of the pretest-posttest design
Mortality (dropout factor)
Assess equivalency of groups with small sample size
Can be used to select participants for the experiment
© 2012 The McGraw-Hill Companies, Inc.

10. BASIC EXPERIMENTS Disadvantages of the pretest-posttest design
Time consuming and awkward to administer
Sensitizes participants to what is being studied
Demand characteristics
Reduces external validity
© 2012 The McGraw-Hill Companies, Inc.

11. © The McGraw-Hill Companies, Inc.

12. ASSIGNING PARTICIPANTS EXPERIMENTAL CONDITIONS
Independent Groups Design
Participants participate in only one group
Repeated Measures Design
Participants are in all conditions
© 2012 The McGraw-Hill Companies, Inc.

13. REPEATED MEASURES DESIGN
Advantages and Disadvantages of Repeated Measures Design
Advantages
Fewer participants
Extremely sensitive to statistical differences
Conditions are identical because person is own control group
Disadvantages
Order effect
Practice effect
Fatigue effect
Carryover effect
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14. REPEATED MEASURES DESIGN
Counterbalancing
Complete counterbalancing
Latin squares
Time Interval Between Treatments
Choosing Between Independent Groups and Repeated Measures Design
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15. COUNTERBALANCING © 2007 The McGraw-Hill Companies, Inc.

16. LATIN SQUARE WITH FOUR CONDITIONS
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17. MATCHED PAIRS DESIGN
Goal is to Match People on a Participant Characteristic
Matched to either the dependent measure or a variable that is strongly related to the dependent variable
© 2012 The McGraw-Hill Companies, Inc.