1. METHODS IN BEHAVIORAL RESEARCH
NINTH EDITION
PAUL C. COZBY
Copyright © 2007 The McGraw-Hill Companies, Inc.

2. CHAPTER 8
EXPERIMENTAL DESIGN

3. LEARNING OBJECTIVES
Describe the relationship between a confounding variable and the internal validity of an experiment
Describe the post-test only design and the pretest-posttest design, including the advantages and disadvantages of each design

4. LEARNING OBJECTIVES
Contrast and 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

5. CONFOUNDING AND INTERNAL VALIDITY
Confounding Variable: confounding occurs when the effects of the independent variable and an uncontrolled variable are intertwined so one cannot determine which is responsible for the effect

6. CONFOUNDING AND INTERNAL VALIDITY
Internal Validity: experiment must be designed and conducted so that only the independent variable can be the cause of the results

7. BASIC EXPERIMENTS Posttest-Only Design Must:
Obtain two equivalent groups of participants
Selection differences
Introduce the independent variable
Measure the effect of the independent variable on the dependent variable

8. POSTTEST-ONLY DESIGN
© 2007 The McGraw-Hill Companies, Inc.

9. BASIC EXPERIMENTS Pretest-Posttest Design
A pretest is given before the experimental manipulation is introduced to make sure groups are equivalent at the beginning of the experiment

10. BASIC EXPERIMENTS Advantages of the pretest-posttest design
Mortality (dropout factor)
Assess equivalency of groups with small sample size
Can use to select participants for the experiment

11. BASIC EXPERIMENTS Disadvantages of the pretest-posttest design
Time consuming and awkward to administer
Sensitize participants to what is being studied
Demand characteristics
Reduces external validity

12. © The McGraw-Hill Companies, Inc.

13. ASSIGNING PARTICIPANTS EXPERIMENTAL CONDITIONS
Independent Groups Design
Participants participate in only one group
Repeated Measures Design
Participants Are in All Conditions

14. 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
- Contrast effect

15. REPEATED MEASURES DESIGN
Counterbalancing
Complete counterbalancing
Latin squares
Time Interval Between Treatments
Choosing Between Independent Groups and Repeated Measures Design

16. COUNTERBALANCING
© 2007 The McGraw-Hill Companies, Inc.

17. LATIN SQUARE

18. MATCHED PAIRS DESIGN
Goal is to Match People on a Participant Characteristic
Either the Dependent Measure or a Variable that is Strongly Related to the Dependent Variable
Analysis of Covariance