QuasiExperimental Research

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Chapter 9

• Quasi-Experimental Research

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Quasi-experiments are almost experiments
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The True Experiment

• Manipulation of a variable
• Control of extraneous variables
• Random assignment of participants to conditions
• Comparison of conditions

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Sometimes the circumstances are such that we
cannot conduct a true experiment. In those
instances we might be able to conduct a
quasi-experiment.
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The Quasi-Experiment

• Will involve a comparison of conditions
• Will not involve
• The manipulation of a variable
• Control of extraneous variables
• Random assignment of participants to conditions

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Some Examples of Quasi-Experiments

• Looking for gender differences in verbal or
  mathematics ability
• Looking for developmental trends among children
• Evaluating the quality of schools

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Inferring Cause and Effect from a Quasi-Experiment
Because quasi-experiments do not involve the
manipulation of an independent variable nor the
control of extraneous variables, it makes it
difficult or impossible to infer cause and effect
from a quasi experiment. Some quasi-experimental
designs are better than others.
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Quasi-experimental designs fall into two general
groups

• Non-equivalent groups designs
• Time series designs

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Non-equivalent groups designs
When the researcher cant control the assignment
of participants to the various groups or
conditions of the study
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1. The Differential Research Design
The researcher compares pre-existing groups that
differ with regard to the variable of interest
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1. The Differential Research Design
The researcher compares pre-existing groups that
differ with regard to the variable of
interest Example 1 Comparing the verbal or
mathematics ability of men and women.
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1. The Differential Research Design
The researcher compares pre-existing groups that
differ with regard to the variable of
interest Example 1 Comparing the verbal or
mathematics ability of men and women. Example 2
Comparing the children from intact and broken
homes.
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The difficulty with differential research
It is impossible to assert that the groups only
differ with regard to the independent variable.
The text refers to this as an assignment bias.
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2. Post-test only non-equivalent control group
design
Following treatment a treatment group is
compared to a non-treatment control group X O
(treatment group) O (control group)
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2. Post-test only non-equivalent control group
design
Following treatment a treatment group is
compared to a non-treatment control group X O
(treatment group) O (control group)
Example evaluation of an educational program.
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Assignment bias is also a problem for the
post-test only nonequivalent control group
design.
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3. Pretest-posttest nonequivalent control group
design
Observations are obtained from both groups prior
to treatment, one group receives the treatment,
then observations are obtained from both groups
again. O X O (treatment group) O O (control
group)
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3. Pretest-posttest nonequivalent control group
design
Observations are obtained from both groups prior
to treatment, one group receives the treatment,
then observations are obtained from both groups
again. O X O (treatment group) O O (control
group)
Example evaluation of a medical or
psychotherapy.
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The pretest-posttest nonequivalent control group
design allows the researcher to check for an
assignment bias prior to the treatment.
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Time Series Designs
A series of observations is made over time of a
single group on individuals
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1. One-group pretest-post test design
Observations are made both before and after a
treatment O X O
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1. One-group pretest-post test design
Observations are made both before and after a
treatment O X O Example Evaluation of a
particular educational program or therapy.
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Time related factors that threaten validity

• History effect external events might have an
  effect
• Instrumentation measurement instrument might
  change over time
• Practice/fatigue effects scores might improve
  from practice or deteriorate from fatigue
• Maturation observed changes might reflect
  physical or psychological maturation
• Regression toward the mean extreme scores on
  the first observation will be less extreme on the
  second

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2. Time series design

• A series of observations are made both prior to
  and following a treatment.
• O O O X O O O

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3. Equivalent time-samples design
O O O X O O O N O O O X O O O N O O O
Works well for treatment effects that are
temporary.
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Developmental Research Designs
Designed to assess the effects of age
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1. Cross-sectional design
Design compares different groups of individuals
that differ with regard to age
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1. Cross-sectional design
Design compares different groups of individuals
that differ with regard to age Example
comparing cognitive abilities of 50-year-olds,
60-year-olds, 70-year-olds, and 80-year-olds
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1. Cross-sectional design
Design compares different groups of individuals
that differ with regard to age Example
comparing cognitive abilities of 50-year-olds,
60-year-olds, 70-year-olds, and
80-year-olds Cohort effects (i.e., differences
between cohorts that are unrelated to age) are a
particular threat to the validity of
cross-sectional designs
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2. Longitudinal designs
Sample same individuals at different points in
time.
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2. Longitudinal designs
Sample same individuals at different points in
time. Example Compare cognitive ability of a
group of individuals when they 50, 60, 70, and 80
years old.
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2. Longitudinal designs
Sample same individuals at different points in
time. Example Compare cognitive ability of a
group of individuals when they 50, 60, 70, and 80
years old. Difficulties with longitudinal designs
mortality (or drop-out rate) and expense.