Experimental Designs

Dr. Farzin Madjidi

Pepperdine University

Graduate School of

Education and Psychology

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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

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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

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Experimental Designs

Single-Group Designs Nonequivalent-Groups Randomized-Groups Factorial Designs Single-Subject Designs

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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

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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

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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

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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

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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

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Randomized-Groups

Posttest Only Subjects randomly assigned to each group

R A X O

B O

Or,

R A X1 O

B X2 O

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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

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Factorial Designs

When more than one independent variable is tested

Method A Method B

Males Group 1 Group 2

Females Group 3 Group 4

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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

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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

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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

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Single Subject DesignsA-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

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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