Small sample research designs for evidence-based rehabilitation: issues and methods

Abstract

Conventional research methods, including randomized controlled trials, are powerful techniques for determining the efficacy of interventions. These designs, however, have practical limitations when applied to many rehabilitation settings and research questions. Alternative methods are available that can supplement findings from traditional research designs and improve our ability to evaluate the effectiveness of treatments for individual patients. The focus on individual patients is an important element of evidenced-based rehabilitation. This article examines one such alternate approach: small-N research designs. Small-N designs usually focus on 10 or fewer participants whose behavior (outcomes) are measured repeatedly and compared over time. The advantages and limitations of various small-N designs are described and illustrated using 3 examples from the rehabilitation literature. The challenges and opportunities of applying small-N designs to enhance evidence-based rehabilitation are discussed.

Figure 1

AB design with follow-up for one person. Y-axis = time to complete the Timed Up and Go test. X-axis = sequence of observations across six baseline (A), six intervention (B), and two follow-up (FU) sessions. The dashed horizontal line indicates the value that is two standard deviations below the mean of baseline phase scores. Adapted from Marklund and Klasbo.

Figure 2

Multiple baseline design across outcomes with follow-up for one person. Y-axis = magnitude of error in perceived joint position. X-axis = sequence of observations across consecutive phases. Vertical lines indicate the transitions from baseline to stimulus-specific intervention, to follow-up phases, respectively. Adapted from Carey and Matyas.

Figure 3

Alternating treatment design with follow-up for one person. Upper chart: Y-axis = step length in meters. X-axis = sequence of observations across consecutive phases. Solid vertical lines indicate phase transitions: baseline, intervention, and follow-up. The lower chart illustrates potential patterns of randomly-assigned intervention sequences applied during each session. Note: The lower chart does not show all 12 alternating intervention sequences. Adapted from Diamond and Ottenbacher.