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Single Subject Research Design(SSRD)

Bill Miller, PhD, OTUBC School of Rehab SciencesVCHA OT Research Scientist

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Objectives

• 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

• 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?

• 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

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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

A-B– 1 baseline + 1 tx phase

Problems– Limited control over

threats to internal validity

Solution Replication– Add phases– Add other tx/interventions– Add subjects

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Withdrawal Designs

A-B-A– Shows intervention leads

to target behaviour (TB)– TB disappears when

intervention stopped

• Problems– Unethical to withdraw tx– Reversible target

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Withdrawal Designs

A-B-A-B- adds an extra tx phase- ↑ confidence in tx effect

Problems- Reversible target

behaviour- Unethical to withdraw tx 0

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Multiple Treatment Designs

Alternating– 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

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Multiple Treatment Designs

Interactive (A-B-BC-B-BC)– Assess separate & joint

effects of 2 or more txs– Best to replicate tx– Interactions can be tested

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Multiple Baseline Designs

Across Subjects/Conditions /Behaviors

• Useful when:– Behaviours non-replicable– Unethical to withdraw tx

• 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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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

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Change in level Change in trendA) B)

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Sample Visual Analyses

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How would you describe the following changes?

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Split-Middle Technique

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SSRD Case Study Between Groups

Subject Own control No control Group control

Hypothesis Test Generate Test Extraneous variable Controls Describes May control

Design flexibility Permits change Permits change No changeDifferences 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

SSRD, Case Study & Group Designs: Contrast and Comparison

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Useful References1. Backman CL, Harris SR. Case studies, single-subject research, and N of 1 randomized trials:

comparisons and contrasts. Am J Phys Med Rehabil 1999;78:170-6.2. 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.

3. Harris SR. Research techniques for the clinician. In B Connolly & P Montgomery (Eds). Therapeutic Exercise in Developmental Disabilities, Hixson, TX: Chattanooga Group.

4. Gonnella C. Single-subject experimental paradigm as a clinical decision tool. Phys Ther 1989;69:601-9.5. 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;314:889-8926. Kazidin AE. Single-case research designs. New York: Oxford University, 1982.7. Portney LG, Watkins MP. Single-case experimental designs. Foundations of clinical research:

applications to practice (2nd ed). Appleton & Lange, 1993.8. 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 187-2489. Wolery M, Harris SR. Interpreting results of single-subject research designs. Phys Ther 1982;4:445.10. Zhan S, Ottenbacher KJ. Single subject research designs for disability research. Disabil Rehabil

2001;23:1-8.

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