Single-System Research Designs by Stephen E. Wong

Single-system designs (SSDs), otherwise known as single-subject, single-case, or N-of-1 designs, are research formats that permit uncontrolled program evaluation and controlled experiments with only one subject, one group, or one system. All SSDs involve intensive study of the individual subject or system through repeated measures over time. Controlled SSDs demonstrate experimental control by manipulating an independent variable and showing corresponding changes in a dependent variable, then replicating manipulation of the independent variable and subsequent change in the dependent variable to demonstrate a cause-and-effect relationship. Replications have been performed through operations such as changing a dependent variable and then reversing that change; producing successive change across different behaviors, settings, or subjects; producing change according to a pre-determined random schedule, or incrementally changing the level of a dependent variable. Emerging from laboratory-based experimental psychology, this methodology has been adopted by applied fields such behavior analysis, clinical psychology, social work, special education, and speech and hearing therapy due its capability to evaluate clinical practice with individual clients who have unique needs and idiosyncratic responses to treatments.

Historical and Conceptual Foundations of Single-System Designs

References in this section show the emergence of SSD methodology from the experimental analysis of behavior to its adoption by applied behavior analysis and clinical psychology; applied behavior analysts still use these designs more frequently than any other human service profession. Sidman 1960 presents the logical framework and types of experimental control in single-system research and contrasts it with statistical control procedures used in between-groups experiments. Moore 1990, in a special issue dedicated to Sidman, reviews these issues and suggests recent movement toward rapprochement between the two approaches. The classic Campbell and Stanley 1963 monograph discusses experimental methodology issues relevant to both SSDs (within-subject) and between-groups designs. Baer, et al. 1968 proposes that SSDs should be the principal research methodology for the nascent field of applied behavior analysis, while Leitenberg 1973 makes a compelling argument for its usefulness in clinical psychology and provides numerous illustrative SSD studies.

• Baer, D. M., M. M. Wolf, and T. R. Risley. 1968. Some current dimensions of applied behavior analysis. Journal of Applied Behavior Analysis 1:91–97. DOI: 10.1901/jaba.1968.1-91 Article defines the behavior change techniques and evaluation strategies of applied behavior analysis (ABA) employing SSDs. Shows the close association between ABA and SSDs, and how these technologies developed and evolved simultaneously.
• Campbell, D. T., and J. C. Stanley. 1963. Experimental and quasi-experimental designs for research. Chicago: Rand McNally. This short text is the definitive explication of internal validity and time-series experiments. It describes the limitations to causal inference in simple, uncontrolled SSDs and how they compare with controlled between-groups research designs.
• Leitenberg, H. 1973. The use of single-case methodology in psychotherapy research. Journal of Abnormal Psychology 82:87–101. DOI: 10.1037/h0034966 Introduces SSDs to clinical psychology and explains how they offer a new way of systematically evaluating clinical practice. Describes most of the major SSDs and provides compelling case illustrations for each of them.
• Moore, Jay. 1990. A special section commemorating the 30th anniversary of Tactics of scientific research: Evaluating experimental data in experimental psychology by Murray Sidman. Behavior Analyst 13:159–161. Introduction to a series of six articles dedicated to the classic Sidman text, plus a reply to the articles by Murray Sidman. Articles examine the book’s profound contribution to the research methodologies of the experimental analysis of behavior and applied behavior analysis, while discussing controversies raised and issues overlooked by the approach.
• Sidman, M. 1960. Tactics of scientific research: Evaluating experimental data in psychology. New York: Basic Books. Presents the conceptual foundation for SSDs as they developed in experimental psychology. Crucial reading to gain a deeper understanding of the logic of SSD methodology. Explains fundamental principles underlying SSDs, including types of replication, experimental control versus statistical control of variability, and the observation and manipulation of steady states of behavior.

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