1. Experimental Designs Dr. Farzin Madjidi Pepperdine University
Graduate School of
Education and Psychology 1

2. Experimental Research
Researcher looks for cause-and-effect relationships
In non-experimental research, the researcher has no control over what has or will happen to the subjects that may affect their response to the dependent variable (treatment).
In experimental research, the researcher can control most of these factors 2 2

3. Experimental Research Types of Control
The researcher has two types of control:
Direct manipulation over of the independent variable by controlling dosages of the treatment (e.g., one group receives the treatment while one doesn’t).
Control over extraneous variables by either keeping constant conditions and events except for the independent variable, or by eliminating them 3 3

4. Experimental Designs Single-Group Designs Nonequivalent-Groups
Randomized-Groups
Factorial Designs
Single-Subject Designs 4 4

5. Conventions Most experimental designs are shown graphically
Letters A,B,C,D are used to represent groups
Letter “O” indicates testing/measurements
Letter “X” indicates intervention/treatment
Letter “R” indicates the subjects were randomly selected/assigned 5 5

6. Single-Group Designs Posttest-Only Designs (pre-experimental)
Inadequate control over extraneous variables
Group Treatment Posttest
A X O
Weakest of experimental designs
Applied when the researchers has reasons to believe pre-intervention factors are benign 6 6

7. Single-Group Designs Pretest-Posttest Designs
Subjects are tested before the intervention
Group Pretest Intervention Posttest
A O X O
No control over extraneous variables
No control/comparison group 7 7

8. Non-Equivalent Groups
Posttest Only Designs
A control/comparison group is added
A X O
B O
No pre-testing to account for initial differences among subjects 8 8

9. Non-Equivalent Groups
Pretest-Posttest Design (Quasi-Experimental)
Best used with existing, intact groups of subjects
A O X O
B O O
Or,
A O X O
B O X O 9 9

10. Randomized-Groups Posttest Only
Subjects randomly assigned to each group
R A X O
B O
Or,
R A X O
B X O 10 10

11. Randomized-Groups Pretest-Posttest Only Designs
Subjects randomly assigned to each group, before or after pretesting
R A O X O
B O O
Or,
R A O X O
B O X O 11 11

12. Factorial Designs When more than one independent variable is tested
Method A Method B
Males Group Group 2
Females Group Group 4 12 12

13. Factorial Designs Males R A O X O B O O Females R A O X1 O B O X2 O
You can replicate any of these designs with more than two groups and more than two interventions 13 13

14. Experimental Designs Key Issues:
Primary purpose is to test causal hypothesis
Direct manipulation of the Independent Variable
Clearly identify the design used
Maximize the control of extraneous variables
Treatment should be sufficiently different 14 14

15. Single Subject Designs
Studies individuals, or individual cases
Involves multiple measures of behavior
Looks for patterns of behavior
Requires a “reliable” measurement instrument
Requires clear descriptions of the conditions of measurement and nature of treatment
Requires at least one baseline and one treatment as a frame of reference
Single variable rule of change 15 15

16. Single Subject Designs
A-B-A Designs
Requires multiple measurements as follows
Baseline Treatment
XXXXXX
OOOOOOOOOOOOOOOOO
Or,
Baseline Treatment Baseline
XXXXXXX
OOOOOOOOOOOOOOOOOOOOOOOOOO
Also called A-B-A design with withdrawal
Multiple-Baseline Designs 16 16

17. Single Subject Design Key Issues Clear definition of target behavior
Reliable measurement of target behavior
Clear, full description of procedures, subjects and setting
Control experimenter/observer effect
Practical significance of results 17 18