methods

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

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

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

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