1. Single Subject Designs

2. Overview Two basic approaches to research:
group designs (between group comparisons)
single subject designs (a.k.a. within subject designs, intra-subject designs, etc.)
Single subject designs originated in behavioral psychology, with “operant conditioning” of rats

3. Single subject designs in Comm. Disorders
limited sample size
hard to obtain large samples with disordered or special populations
small samples result in low statistical power when using between group designs
sometimes the sample consists of only one or two clients
need for extensive or repeated observations
more time consuming to run subjects individually
harder to do detailed, in-depth observations

4. The basic concept of single subject designs
Each subject serves as his/her own treatment and control condition
The subject alternates between the treatment and control conditions
changes in the subject’s speech/language performance are compared across the various treatment and control conditions
often, “longitudinal” or “time-series” designs are used (employing ongoing measurements at time 1, time 2, time 3, etc.)

5. Nomenclature for single subject designs
Phases or stages of the experiment are typically designated by letters
A phase: initial baseline performance; preliminary diagnosis or assessment before any treatment.
B phase: treatment condition. therapy is initiated for a specified period of time.
A phase (2nd): withdrawal phase. Therapy is withdrawn to see if the client’s speech/language performance decays.
C phase: alternative treatment condition; a second type of therapy is introduced.

6. Types of Single Subject Designs
Descriptive designs (not true experiments)
A only design: or case study: observation only, no manipulation, no control. A client is observed, speech/language samples are taken, no therapy is provided.
B only design: treatment or therapy only, no baseline measures, no control. A client receives treatment, but no speech/language assessment is performed prior to beginning treatment
AB design: baseline measures are taken, followed by the introduction of therapy. However, control is lacking (improvements could be due to spontaneous recovery, history, maturation, etc.)

7. More Types of Single Subject Designs
Quasi-experiments
ABA design: a stable baseline is established, followed by the onset of therapy, followed by the withdrawal of therapy.
Speech should
improve during
the treatment phase
Speech should
regress during
the withdrawal
phase A B A

8. More Types of Single Subject Designs
Quasi-experiments
ABAB design: a stable baseline is established, followed by the onset of therapy, followed by the withdrawal of therapy.
Speech should
improve during
the treatment phase
Speech should
regress during the
withdrawal phase
Speech should
improve again
with the
reintroduction
of treatment A B A B

9. More complicated designs
ABABA reversal:
Instead of reintroducing the treatment during the second B phase, treatment is reversed. That is, incorrect speech/language is reinforced.
ABACA design:
Instead of the second B phase, an alternative treatment condition, C, is introduced. The researcher wants to know if speech/language improves more during the B or C phase.
Note: many other combinations are possible

10. Matched pairs design Used when a researcher has 10-15 subjects
more than enough subjects for a single-subject design, yet still too few for a true between-groups design
Subjects are rank ordered on the basis of their assessments on some speech/language variable
Example: 12 stutterers are rank ordered 1 through 12 according to stuttering severity.
Subjects are then paired off with their most similar counterparts
In order to get the pairing precise enough, a researcher may recruit partners outside of the study.
Example: stutterers 1 and 2 form a matched pair, stutterers 3 and 4 form a matched pair…etc.
Comparisons are then made between the performances of the matched pairs

11. Illustration of a matched pairs design
12 Stutterers are rank ordered according to stuttering severity
Biff
Rudolpho
Rex
Otto
Fifi
Zack
Muffin
Tad
Dieter
Lulu
Raheem
Felix
Matched pairs
Treatment Control
Biff Rudolpho
Otto Rex
Fifi Zack
Tad Muffin
Dieter Lulu
Felix Raheem

12. Matched groups design
A “between groups” design, that avoids random assignment in favor of matching on specific, predefined criteria
A matched groups design is similar to matched pairs.
Instead of matching one individual with another, the two groups are matched with each other.
Groups may be matched on multiple criteria (sex, age, language, etc.)
The researcher compares the average performance of one group with the other, as opposed to the performance of one individual with his/her matched counterpart.