Single Subject Research Design
(SSRD)
Bill Miller, PhD, OT
UBC School of Rehab Sciences
VCHA OT Research Scientist
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Objectives
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Define Single Subject Research Design (SSRD)
Rationale for using SSRD
Describe several forms of SSRD
Discuss the methods of analyses using SSRD
Discuss strengths & weaknesses of SSRD
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Continuum of Research
Quasi-Experimental
(Groups -no random selection)
Case report
Case Study
(Individuals - SSRD)
Experimental
(RCT)
Pre Scientific
Scientific
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Other Names for SSRD
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Single case experimental design
Time series design
Small-N design
Single system designs
Within-subject comparison
Idiographic research
N of one trial
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SSRD Defined
SSRD involves studying a single individual or
system by taking repeated measurements of 1
or more dependent variables and
systematically applying & sometimes,
withdrawing or varying the independent
variable.
(Ottenbacher, 1986; Bloom & Fischer, 1982)
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Why Chose SSRD?
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Demonstrates individual differences
Difficulty obtaining adequate power (N)
Difficulty obtaining homogeneous group
SSRD is relatively easy to do
Helps validate practice
Demonstrate treatment (tx) effectiveness
Great pilot
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About SSRD
• Collection of methods (eg; AB, ABAC)
• Answers research question or tests hypothesis
• Involves at least one;
• Subject (individual or clinical unit/department)
• Baseline or “A” Phase - series of repeated observations
or measurements with no intervention
• Intervention or “B” Phase - introduction of experimental
variable with evaluation to see if change occurs
• Dependent variable (quantifiable)
• Independent variable (treatment / intervention)
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SSRD Phases
Baseline
• Period of no tx - reflects natural state
• Provide standard for evaluating tx effect
• Measurements usually repeated until stability
demonstrated
• 5 or more data points
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Sample Baseline Phases
93.5
A)
12
B)
93
10
92.5
Behaviour
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6
4
92
91.5
91
90.5
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90
0
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89.5
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Se s s i o ns
93
92.5
C)
93
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D)
92
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91
Behaviour
Behaviour
91.5
91
90.5
90
90
89
89.5
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88.5
87
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SSRD Phases
Intervention Phase
• Introduce tx or intervention
• = independent variable
• Multiple treatments are alright
• Repeated measurement of dependent variable
• Phase length should be approximately same
as baseline
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Dependent Variable
• Variable of interest needs to be quantifiable
• Does not require a standardized test
• Usually frequency, duration, magnitude
• i.e. episodes of self-abuse / walk time / pain intensity
• Should assess reliability of variable
» total % agreement
» Point by point % agreement
» kappa
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Simple Design
12.5
A-B
11.5
– 1 baseline + 1 tx phase
10.5
Problems
– Limited control over
threats to internal validity
units
9.5
8.5
7.5
Solution
Replication
6.5
– Add phases
– Add other tx/interventions
– Add subjects
5.5
A
B
4.5
1
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Time (session)
12
Withdrawal Designs
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A-B-A
8
• Problems
7
Units
– Shows intervention leads
to target behaviour (TB)
– TB disappears when
intervention stopped
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5
– Unethical to withdraw tx
– Reversible target
behaviour
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A
B
A
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Time (sesions)
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Withdrawal Designs
70
A-B-A-B
60
- adds an extra tx phase
- ↑ confidence in tx effect
B
A
B
50
# of hits
Problems
- Reversible target
behaviour
- Unethical to withdraw tx
A
40
30
20
10
0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
Days
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Multiple Treatment Designs
96
Alternating
Oxygen staturation
– Rapid alternation of ≥ 2 txs (or tx &
placebo)
– Txs alternated within same session,
session to session
– Advantage = quick results, baseline
unnecessary
– Sequence effect randomizing
counterbalance
– Target must be clear & happen
rapidly
A
B
95
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90
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Daily sessions
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Multiple Treatment Designs
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Interactive (A-B-BC-B-BC)
Weight (Kg) and Calories
– Assess separate & joint
effects of 2 or more txs
– Best to replicate tx
– Interactions can be tested
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A
B
BC
B
BC
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Days
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Multiple Baseline Designs
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Across Subjects
/Conditions /Behaviors
Eye Fixation
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• Useful when:
0
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– Behaviours non-replicable
– Unethical to withdraw tx
15
Facial Posturing
10
• Uses basic A-B phase design
– Measure baseline in all
– Establish stability
– Intro tx to 1st
subject/condition…
– Intro next tx when previous tx
stabilizes
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Diaphragmatic
Breathing
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SSRD Analysis
• Visual analysis most common
• Assess trends & levels between adjacent phases
– Level – refers to change in value or magnitude of
dependent variable after intervention
– Trend – refers to change in direction
– Described as accelerating, decelerating, stable or variable
• Another technique Split middle Difference
– Uses celeration line to which statistical significance can be
explored
– Also 2 standard deviation method
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Visual Analysis
12.5
A)
Change in level
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11.5
B)
Change in trend
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10.5
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9.5
Units
units
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8.5
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7.5
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6.5
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5.5
0
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Time (sesion)
Time (session)
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Sample Visual Analyses
How would you describe the following changes?
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9.5
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Units
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7.5
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0
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15
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Time (sesion)
Sessions
12
12
10
10
8
8
Units
Units
Behaviours
8.5
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Time (sesion)
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Time (sesion)
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Split-Middle Technique
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A
B
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SSRD, Case Study & Group Designs: Contrast and
Comparison
SSRD
Case Study
Between Groups
Subject
Own control
No control
Group control
Hypothesis
Test
Generate
Test
Describes
May control
Extraneous variable Controls
Design flexibility
Permits change Permits change
No change
Differences
Individual
Individual
Group
Generalizability
Limited
None
Similar to groups
Measurement
Repeated freq Through-out
Few
Cost
Low
Low
Can be high
Statistics
Limited
None
Significance bt grps
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Useful References
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Backman CL, Harris SR. Case studies, single-subject research, and N of 1 randomized trials:
comparisons and contrasts. Am J Phys Med Rehabil 1999;78:170-6.
Backman CL, Harris SR, Chisholm JM, Monette AD. Single-subject research in rehabilitation: a review
of studies using AB, withdrawal, multiple baseline and alternating treatments designs. Arch Phys Med
Rehabil 1997;78:1145-53.
Harris SR. Research techniques for the clinician. In B Connolly & P Montgomery (Eds). Therapeutic
Exercise in Developmental Disabilities, Hixson, TX: Chattanooga Group.
Gonnella C. Single-subject experimental paradigm as a clinical decision tool. Phys Ther 1989;69:601-9.
Guyatt G, Sackett D, Taylor W, Chong J, Roberts R, Pugsley S: Determining optimal therapy:
randomized trials in individual patients. N Engl J Med 1986;314:889-892
Kazidin AE. Single-case research designs. New York: Oxford University, 1982.
Portney LG, Watkins MP. Single-case experimental designs. Foundations of clinical research:
applications to practice (2nd ed). Appleton & Lange, 1993.
Sackett DL, Haynes RB, Guyatt GH, Tugwell P: Deciding on the best therapy, in Clinical Epidemiology:
A Basic Science for Clinical Medicine, ed 2. Boston, Little, Brown & Co., 1991, pp 187-248
Wolery M, Harris SR. Interpreting results of single-subject research designs. Phys Ther 1982;4:445.
Zhan S, Ottenbacher KJ. Single subject research designs for disability research. Disabil Rehabil
2001;23:1-8.
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