Experimental Designs

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

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

3. Experimental ResearchTypes 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

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

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

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

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

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 X1 O B O X2 O 9

10. Randomized-Groups Posttest Only • Subjects randomly assigned to each group R A X O B O Or, R A X1 O B X2 O 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 X1 O B O X2 O 11

12. Factorial Designs • When more than one independent variable is tested Method A Method B Males Group 1 Group 2 Females Group 3 Group 4 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

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

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

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

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