Small N designs for rehabilitation research.

Rehabilitation research presents unique and challenging problems to investigators during both the design and analysis periods. Statistical issues regarding sample size requirements for an adequately powered study may be in direct conflict with realistic recruitment and subject retention goals. Issues of underpowered studies, sample size requirements, and recruitment goals plague rehabilitation research. Randomized clinical trials (RCTs) are typically narrow in scope and thus lack generalizability to everyday, yet specific, clinical problems; they are also costly and time-consuming and require large numbers of participants for randomization to have optimal, desired effects. Further, the RCT design may not be applicable to assistive technologies and environmental modifications-vital components of disability and rehabilitation research-nor is it appropriate in situations in which theoretical models of change are lacking or premature. Single-case designs are better suited for studies in which understanding and changing patient behavior and functional status are primary goals and the targeted sample sizes are less than 30 and frequently less than 10. Theoretical, methodological, and clinical reasons for using experimental and quasi-experimental single-case designs are presented. Recommendations for designing and conducting single-case studies that contribute to the evidence base are also discussed.