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© 2011 Pearson Education, Inc. All rights reserved.
The (Never-Ending) Search for Causation
• Establishing causation among variables :
» Produces increased understanding of those variables
» Results in the ability to manipulate conditions in
order to produce desired changes
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Experimental Research
• Can demonstrate cause-and-effect very convincingly
• Very stringent research design requirements
• Experimental design requires:
» Randomly selected participants who are randomly
assigned to groups (experimental and control)
» Independent treatment variable that can be applied
to the experimental group
» Dependent variable that can be measured in all
groups
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Quasi-Experimental Research
• Used in place of experimental research when random
assignment to groups is not feasible
• Otherwise, very similar to true experimental research
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Fundamentals of Experimental and
Quasi-Experimental Research
Cause and effect:
» Incorporates a temporal element—the cause is a
condition that exists prior to the effect; effect is a
condition that occurs after the cause
» There exists a “logical connection”—cause-and-effect
is demonstrated when manipulation of the
independent variable results in differences in the
dependent variable (as evidenced by comparing the
experimental group to the control group)
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Fundamentals of Experimental and
Quasi-Experimental Research (cont’d.)
Random selection and random assignment :
» Distinguish between “selection” and “assignment”
» Both help to ensure that groups are equivalent and
to control for extraneous variables
» If you incorporate random selection and random
assignment
Experimental research
» If you incorporate random selection but not random
assignment
Quasi-experimental research
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Fundamentals of Experimental and
Quasi-Experimental Research (cont’d.)
When to use experimental research design :
» If you strongly suspect a cause-and-effect
relationship exists between two conditions, and
» At least randomly assigned group is available to
participate, and
» The independent variable can be introduced to
participants and can be manipulated, and
» The resulting dependent variable can be measured
for all participants
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© 2011 Pearson Education, Inc. All rights reserved.
Fundamentals of Experimental and
Quasi-Experimental Research (cont’d.)
Commonly used experimental design notation :
X1 = treatment group
X2 = control/comparison group
O = observation (pretest, posttest, etc.)
R = random assignment
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Common Experimental Designs
Single-group pretest-treatment-posttest design:
O
X
O
» Technically, a pre-experimental design (only one
group; therefore, no random assignment exists)
» Overall, a weak design—no control group (for
comparison purposes); pretest sensitization;
experimenter effects (therefore, cannot be sure
about conclusions)
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Common Experimental Designs (cont’d.)
Two-group treatment-posttest-only design:
R
R
X1
X2
O
O
» Here, we have random assignment to experimental,
control groups
» A better design, but still weak—cannot be sure that
groups were equivalent to begin with
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© 2011 Pearson Education, Inc. All rights reserved.
Common Experimental Designs (cont’d.)
Two-group pretest-treatment-posttest design:
R
O
X1
O
R
O
X2
O
» A substantially improved design—previously
identified errors have been reduced
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Common Experimental Designs (cont’d.)
Solomon four-group design:
R
O
X1
O
R
O
X2
O
R
X1
O
R
X2
O
» A much improved design—how??
» One serious drawback—requires twice as many
participants
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Common Experimental Designs (cont’d.)
Factorial designs:
R
O
X1
g1
O
R
O
X2
g1
O
R
O
X1
g2
O
R
O
X2
g2
O
» Incorporates two or more factors
» Enables researcher to detect differential differences
(effects apparent only on certain combinations of
levels of independent variables)
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Common Quasi-Experimental Designs
Posttest-only design with nonequivalent groups:
X1
O
X2
O
» Uses two groups from same population
» Questions must be addressed regarding
equivalency of groups prior to introduction of
treatment
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Common Quasi-Experimental
Designs (cont’d.)
Pretest-posttest design with nonequivalent groups:
O
X1
O
O
X2
O
» A stronger design—pretest may be used to establish
group equivalency
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Common Quasi-Experimental
Designs (cont’d.)
Time series designs:
O
O
O
O
X
O
O
O
O
» Uses a single group of participants
» Examines possible changes over time
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Similarities Between Experimental
and Quasi-Experimental Research
• Cause-and-effect relationship is hypothesized
• Participants are randomly selected and randomly
assigned (experimental) or randomly selected and
nonrandomly assigned (quasi-experimental)
• Application of an experimental treatment by researcher
• Following the treatment, all participants are measured
on the dependent variable
• Data are usually quantitative (exception is singlesubject designs); analyzed by looking for significant
differences on the dependent variable
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Single-Subject Designs
• Similar to experimental and/or quasi-experimental designs
(i.e., essentially having a pretest, an intervention, and a
posttest), but with only one participant
• Basic single-subject design:
O
O
O
|
X
O
X
O
|
O
O
O
» Purpose is to monitor effects on one subject
» Results can be generalized only with great caution
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Single-Subject Designs (cont’d.)
Variations of the basic single-subject design include:
» AB design
» ABA design (a.k.a. “reversal design”)
» Alternating treatment design
» Multiple baseline design
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Internal and External Validity
• “Validity of research” refers to the degree to which the
conclusions are accurate and generalizable
• Both experimental and quasi-experimental research are
subject to threats to validity
• If threats are not controlled for, they may introduce
error into the study, which will lead to misleading
conclusions
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Threats to Internal Validity
Internal validity—extent to which differences on the
dependent variable are a direct result of the
manipulation of the independent variable
» Differential selection of participants—participants
are not selected/assigned randomly
» History—when factors other than treatment can
exert influence over the results; problematic over
time
» Maturation—when changes occur in dependent
variable that may be due to natural developmental
changes; problematic over time
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Threats to Internal Validity (cont’d.)
» Testing—also known as “pretest sensitization”;
pretest may give clues to treatment or posttest and
may result in improved posttest scores
» Attrition (mortality)—loss of participants
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Threats to External Validity
External validity—extent to which the results can be
generalized to other groups or settings
» Population validity—degree of similarity among
sample used, population from which it came, and
target population
» Personological variables—application of findings
based on individual differences
» Ecological validity—physical or emotional situation
or setting that may have been unique to the
experiment
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Published Example of QuasiExperimental Research
“The Influence of Computer-Assisted Instruction on Eighth
Grade Mathematics Achievement”
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Published Example of Single-Subject
Research
“A Language Programme to Increase the Verbal Production
of a Child Dually Diagnosed with Down Syndrome and
Autism”
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© 2011 Pearson Education, Inc. All rights reserved.
Applying Technology…
Web sites related to experimental and quasiexperimental research
• Dr. Trochim’s discussion of "Experimental Designs"
(socialresearchmethods.net/kb/desexper.htm)
• Dr. Trochim’s discussion of "Two-Group Experimental Designs"
(socialresearchmethods.net/kb/expsimp.htm)
• Dr. Trochim’s discussion of "Factorial Designs"
(socialresearchmethods.net/kb/expfact.htm)
• Dr. Trochim’s discussion of "Randomized Block Designs"
(socialresearchmethods.net/kb/expblock.htm)
• Dr. Trochim’s discussion of "Covariance Designs"
(socialresearchmethods.net/kb/expcov.htm)
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© 2011 Pearson Education, Inc. All rights reserved.
Applying Technology…
More web sites related to experimental and quasiexperimental research
• Dr. Trochim’s discussion of "Hybrid Experimental Designs"
(socialresearchmethods.net/kb/exphybrd.htm)
• Dr. Trochim’s examination of quasi-experimentation
(socialresearchmethods.net/kb/quasiexp.htm)
• Dr. Trochim’s discussion of "The Nonequivalent Control Group
Design" (socialresearchmethods.net/kb/quasnegd.htm)
• Dr. Trochim’s discussion of "Other Quasi-Experimental Designs"
(socialresearchmethods.net/kb/quasioth.htm)
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© 2011 Pearson Education, Inc. All rights reserved.