Small N Designs for Rehabilitation Research

By Barnett, Scott D.; Heinemann, Allen W. et al. | Journal of Rehabilitation Research & Development, January-February 2012 | Go to article overview

Small N Designs for Rehabilitation Research


Barnett, Scott D., Heinemann, Allen W., Libin, Alexander, Houts, Arthur C., Gassaway, Julie, Sen-Gupta, Sunil, Resch, Aaron, Brossart, Daniel F., Journal of Rehabilitation Research & Development


INTRODUCTION

Rehabilitation research implements studies in which the long-term goals are to improve health and promote wellness for persons with physical disabilities. Rehabilitation presents a growing area of research within society as a whole and, more specifically, within the Veterans Health Administration (VHA) and Department of Defense. The recent Operation Iraqi Freedom/Operation Enduring Freedom conflicts have resulted in an unprecedented number of wounded warriors presenting for rehabilitation because of traumatic brain injury (TBI), blast injuries, amputation, and other conditions, which frequently include polytrauma characterized by lung, bowel, and inner ear injuries; traumatic-limb or partial-limb amputation; soft tissue trauma from fragments and other missiles; and posttraumatic stress injuries [1-3]. As a result, the VHA has targeted rehabilitation research as a primary focus of the overall research portfolio.

The randomized clinical trial (RCT) is the gold standard of research designs, providing the best evidence of effect [4]. The RCT is regarded as the most rigorous design because of the prospective nature, randomization of subjects to independent study arms, and blinding process. Ideally, the randomization process balances potential confounding factors equally across study groups, and blinding reduces potential bias by blocking investigators and subjects from the hypothesis under investigation.

However, RCTs are generally narrow in scope and thus lack generalizability [5-7]; they are costly and time-consuming. The RCT design may not be applicable to assistive technologies and environmental modifications--vital components of disability and rehabilitation research. In many clinical scenarios, a meaningful control group experience is difficult--if not impossible--to design or implement. For example, many interventions in the rehabilitation setting are highly individualized (e.g., modifying assistive equipment to individual needs or abilities) and a control-group comparison is unreasonable [8]. RCTs are typically contingent on participants consenting to the randomization process, which raises concerns about the degree to which consenting participants are representative of the larger number of those who are unable or unwilling to consent. Rehabilitation research often involves specific behavioral and performance outcomes among persons who have low-incidence conditions or who have multiple and complex co-occurring conditions (e.g., polytrauma with behavioral disturbance among returning veterans). The effectiveness of randomization depends on large samples, representative of the population of concern, to distribute unmeasured factors that might otherwise influence results.

Issues of underpowered studies, sample size requirements, and recruitment goals often plague rehabilitation research. Statistical issues regarding sample size requirements for an adequately powered RCT may be in direct conflict with realistic recruitment and subject retention goals. There is simply no margin for error given the number of available subjects with infrequent or co-occurring conditions. Rehabilitation researchers are hard-pressed to balance scientific rigor with clinical feasibility. Consequently, the narrow scope and stringent requirements of the RCT may be theoretically premature, clinically time-consuming, and of questionable generalizability for many research problems encountered in clinical rehabilitation.

In January 2010, the Department of Veteran Affairs (VA), Rehabilitation Research and Development Service convened a State-of-the-Art conference in Miami, Florida, to discuss current and future seminal issues pertinent to rehabilitation research both within and without VA. In this article, we summarize the strategy discussion about situations typically encountered by rehabilitation researchers in which small samples sizes are an issue. Quasi-experimental and experimental small N designs are ideal methods for clinical research in which understanding and changing maladaptive patterns in a patient's behavior and functional status are primary goals [9-10]. …

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