Experimental
Designs
Leedy and Ormrod, Ch. 10
Introduction
Experiments are part of the traditional
science model
 Involve taking “action” and observing
consequences of this action
 Can collect data using rigorous control
 A good example of a sophisticated
experiemntal design experiment is Albert
Bandura’s Bobo doll study: “Transmission
of Aggression Through Imitation of
Aggressive Models” (1961)


Go to this link to read this classic paper:
http://psychclassics.yorku.ca/Bandura/bobo.htm
Introduction (cont.)
Excellent for hypothesis testing and
establishing causality
 For explanatory research
 Small group interaction
 Two types:

 Laboratory

Used in psychology
 Natural

settings (traditional)
settings (field experiments)
More typical in sociology
The classical experiment

Has three pairs of components:
 Independent
and dependent variables
IV is manipulated
 DV is observed for change

 Pre-testing

and post-testing
To compare variation in DV before and after
treatment
 Experimental

and control groups
Experimental group receives “treatment” and is
compared to control group (no treatment)
Experimental Subjects
Probability sampling if possible
 Random assignment essential to ensure
no difference between experimental and
control groups
 Sometimes “matching” used

 Individuals
are matched on important
characteristics

Problem of “volunteer” subjects
“Pre-experimental” Designs
No random assignment used
 A. One shot case study:

Tx (IV)

B. One group pre-test-post-test design:
Obs1

Obs (DV)
Tx
Obs2
C. Static group comparison:
Exp. Grp.
Ctrl. Grp.
Tx
Obs
Obs
“True Experimental” Designs
Use random assignment
 D. Pretest-Post-test control group design
(classical experimental design):

Exp.
Ctrl.

Obs1
Obs1
Tx
Obs2
Obs2
E. Post-test only control group design
Exp.
Ctrl.
Tx
Obs
Obs
“True Experimental” Designs (cont.)

F. Solomon Four-Group design (Bandura’s
Design):
Exp.
Ctrl.
Exp.
Ctrl.
Obs1
Obs1
Tx
Tx
Obs2
Obs2
Obs2
Obs2
Problems of Internal Validity

Is change in DV actually caused by IV?

A. Problems concerning Subjects
 1.
Selection bias
 2. Experimental Mortality
 3. Rivalry
 4. Demoralization
Problems (cont.)

B. Problems with experimental procedure
 1.
Testing
 2. Instrumentation
 3. Causal time order
 4. Diffusion
 5. Compensation to control group

C. Problems with time
 History
 Maturation
The Double-Blind Experiment

Neither researchers or subjects know who
is experimental group

To reduce experimental bias
The Hawthorne Effect:



The Hawthorne effect – refers to change in
behaviour produced by the “stimulus” of being
singled out and made to feel important during
the experimental process.
First observed 1925-1928 at Hawthorne plant in
Illinois during series of experiments on effects of
lighting on worker productivity
Explanation for increase provided by Professor
Elton Mayo (“founder” of human relations theory)
The Hawthorne Experiments

Four experiments:
 A.
Used Three experimental groups and no control
group. Found productivity increased when lighting
improved.
 B. Experimental and control group used.
Experimental group had increased lighting. Both
groups showed increased productivity.
 C. Experimental and control group used.
Experimental group had decreased lighting. Both
groups showed increased productivity.
 D. Two women studied under varying lighting
conditions. Subjects increased productivity according
to which lighting conditions (more or less) researchers
said were “good”
Other Problems:
External validity or generalizability of
experimental research
 Modelling Effect
 Sample problems

 Volunteers
vs non-volunteers have different
characterisitics
Strengths and Weaknesses of
Experimental Design

Strengths
 Establishing
causality
 Control
 Longitudinal
research
 Replication

Weaknesses
 Artificiality
 Experimenter
 Sample
size
effect
The Field Experiment
Used in sociology
 Natural setting
 Uses statistical control
 For an excellent example of a field
experiment, read “The Impact of
Television: A Longitudinal Canadian Study”
by Tannis Macbeth Williams

 To
read a summary of this work, go to:
http://world.std.com/~jlr/comment/tv_impact.htm
Strengths and Weakness of
Field Experiments

Strengths
 Takes
place in “real world”
 Very good generalizability

Weaknesses
 Lack
of internal validity
 Difficult to establish causality
 Time consuming
 Subject to researcher bias