Chapter 12
Quasi-Experimental and Single-Case Designs
Definition
 The quasi-experimental approach is used when it is
impossible to randomly assign participants to
comparison groups and when a researcher is faced
with a situation where only one or two participants
can participate in the research study (single case
designs).
Reliability is weaker
 Causal explanations can be made when using quasi-
experimental designs but only when you collect data
that demonstrate that plausible rival explanations
are unlikely, and the evidence will still not be as
strong as with experimental
Three quasi-experimental designs
 The nonequivalent comparison-group design
 The interrupted time-series design
 The regression discontinuity design
Nonequivalent Comparison-Group Design
 Contains a treatment group and a nonequivalent
untreated comparison group which are administered
pretest and posttest measures.
 The groups are “nonequivalent” because you lack
random assignment
 Only when you can rule out the effects of confounding
variables can you confidently attribute the observed
group difference at the posttest to the independent
variable.
Example?
Interrupted Time-Series Design
 Compares pretest and posttest responses obtained from a
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single group of participants.
Participants are pretested and posttested a number of times
The pretesting phase is called the baseline
A treatment effect is demonstrated only if post-treatment
responses differs from the pretreatment responses.
The main potentially confounding variable that cannot be
ruled out is a history effect. The history threat is a plausible
rival explanation if some event other than the treatment cooccurs with the onset of the treatment.
Example?
Regression Discontinuity Design
 Looks for a discontinuity in regression lines between
individuals who score lower and higher than some
predetermined cutoff score on an assignment variable.
 It is a design used to determine if individuals meeting
some predetermined criteria profit from receiving some
special treatment.
For example:
 Use a standardized test with a cutoff at 50
 Administer treatment to those scoring in pretest above
50
 Administer posttest to both groups, and see if there is a
discontinuity in the regression line. If there is none, the
treatment didn’t work.
 This is actually quite a strong design, and
methodologists have, for a number of years, been trying
to get researcher to use this design more frequently.
 Figure 12.9 on pg 340 clarifies
Control
Treatment
50
Control
Treatment
50
4 Single-Case Experimental Designs
 Treats single participants, one at a time. Or one single
group of individuals.
1. A-B-A design,
2. A-B-A-B design,
3. Multiple-baseline design
4. Changing-criterion design
A-B-A and A-B-A-B Designs
 The A-B-A design - participant is repeatedly pretested
(the first A phase), then the experimental treatment
condition is administered and the participant is
repeatedly post-tested (the B phase)
 Finally, the treatment is stopped and participant is tested
again (second A phase)
 The effect of the treatment is demonstrated if the
pattern differs from A to B then reverts back to A
For example,
 Student talks out of turn frequently (A)
 Teacher administers treatment (B)
 Teacher stops administering treatment (A)
 If student returns to talking out of turn frequently, treatment
was effective.
 One limitation of the A-B-A design is that it ends with
baseline condition at the end of the experiment.
 This limitation can be overcome by including a fourth phase
which adds a second administration of the treatment
condition so the design becomes an A-B-A-B design.
Multiple-Baseline Design
 This design focuses on two or more different behaviors
in the same individual
 Or on the same behavior exhibited by two or more
individuals
 Or on the same behavior exhibited by one individual but
in different settings
Example: Same behavior in four people
 Step 1: Get baseline pretest on all four people
 Step 2: Treat first person only, don’t treat the other three
 Step 3: Treat second person only
 Step 4: Treat third person only
 Step 5: Treat fourth person only
If results show change during treatment, treatment is
effective
A polluting variable
 The other three may change behavior because one of the
four is changing.
 For example, if all four talk out of turn, and one is
silenced through treatment, the other three may grow
quiet because one is silent, not because of the treatment
Changing-Criterion Design
 This is a single-case design that is used when a behavior
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needs to be shaped over time or when it is necessary to
gradually change a behavior through successive
treatment periods to reach a desired criterion.
Step 1: Get baseline
Step 2: Administer treatment
Step 3: Administer stronger treatment
Step 4: Administer even stronger treatment
end