CHAPTER 11: SINGLE CASE, QUASI-EXPERIMENTAL, AND DEVELOPMENTAL
This chapter focuses on three types of special research situations
1.The instance in which the effect of an independent variable must be inferred from an
experiment with only one participant- single case experimental designs.
2.Describing pre-experimental and quasi-experimental designs that may be
considered if it is not possible to use one of the true experimental designs described
in chapter 8
3.Considering research designs for studying changes that occur with age.
SINGLE CASE EXPERIMENTAL DESIGNS
DEFINITION: An experiment in which the effect of the independent variable is assessed
using data from a single participant.
•Single case experimental designs have traditionally been called single-subject
designs, but now the terms used are single case and single participant.
•EARLIER INTEREST: In psychology came from research on reinforcement
schedules pioneered by B.F Skinner
•TODAY: Research using single case designs are often seen in clinical, counseling,
educational, and other applied settings.
•Were developed from a need to determine whether an experimental manipulation
had an effect on a single research participant.
•HOW THE SINGLE CASE EXPERIMENT IS MEASURED?
oSubject’s behavior is measured over time during a baseline control period.
Baseline: in a single case design, the subject’s behavior during a
control period before introduction of the experimental manipulation.
oThe manipulation is then introduced during a treatment period, and the
subject’s behavior from baseline to treatment periods is evidence for the
effectiveness for manipulation.
oPROBLEM: There could be many explanations for the change other than the
experimental treatment (i.e., alternative explanations)
Example: some other event may have coincided with the introduction
of the treatment.
DEFINITION: a single case design in which the treatment is introduced after a baseline
period and then withdrawn during a second baseline period. It may be extended by adding a
second introduction of the treatment. Sometimes called a “withdrawal” design.
•The basic issue in single case experiments is how to determine that the
manipulation of the independent variable had an effect.
oOne method is to demonstrate the reversibility of the manipulation
oREVERSAL DESIGN takes the following form,
•A (baseline period) B (treatment period) A
•This design is called an ABA
•A –behavior be observed during the baseline control period and
again during the treatment
•B – behavior be observed during the period and also during a
•A - behavior be observed during period after the experimental
treatment has been removed.
•Sometimes this is called a withdrawal design, in
recognition of the fact that the treatment is removed or
•EXAMPLE: The effect of a reinforcement procedure on a child’s
academic performance could be assessed with an ABA design.
•The number of correct homework problems could be
measured each day during the baseline(A)
•A reinforcement treatment procedure would then be
introduced in which the child received stars for correct
problems; the stars could be accumulated and
exchanged for toys and candy. (B)
•Later, this treatment would be discontinued during the
second baseline period (A)
*THE FACT THAT BEHAVIOUR CHANGED WHEN
THE TREATMENT WAS INTRODUCED AND
REVERSED WHEN THE TREATMENT WAS
WITHDRAWN IS EVIDENCE FOR ITS
•Some treatments do produce an immediate change in behavior, but many
other variables may require a longer time to show an impact.
•Dermer and Hoch (1999) point out that single case designs are appropriate
for these variables by maintaining a long treatment period.
•The ABA design can be greatly improved by extending it to and ABAB design
in which the experimental treatment is introduced a second time, or even an
ABABAB design that allows the effect of the treatment to be tested a third
oThis is done to address TWO PROBLEMS WITH THE ABA
•A single reversal is not extremely powerful evidence for the
effectiveness if the treatment.
•The observed reversal might have been due to random
fluctuation in the child’s behavior; perhaps the
treatment happened to coincide with some other event
(child’s upcoming birthday) that caused the change ( and
the post-birthday reversal)
•These possibilities are much less likely if the treatment
has been shown to have an effect two or more times;
random or coincidental events are unlikely to be
responsible for both reversals.
•The second problem is ETHICAL.
•As Barlow and Hersen (1984) point out, it doesn’t seem
right to end the design with the withdrawal of a
treatment that may be very beneficial for the
•Using an ABAB design provides the opportunity to
observe a second reversal when the treatment is
introduced again. The sequence ends with the treatment
rather than the withdrawal of the treatment.
•A control group also may be used in reversal design.