Background: There is a growing body of research that evaluates interventions for neuropsychological impairments using single-case experimental designs and diversity of designs and analyses employed. Aims: This paper has two goals: first, to increase awareness and understanding of the limitations of therapy study designs and statistical techniques and, second, to suggest some designs and statistical techniques likely to produce intervention studies that can inform both theories of therapy and service provision.
Main Contribution & Conclusions:We recommend a single-case experimental design that incorporates the following features. First, there should be random allocation of stimuli to treated and control conditions with matching for baseline performance, using relatively large stimulus sets to increase confidence in the data. Second, prior to intervention, baseline testing should occur on at least two occasions. Simulations show that termination of the baseline phase should not be contingent on "stability." For intervention, a predetermined number of sessions are required (rather than performance-determined duration). Finally, treatment effects must be significantly better than expected by chance to be confident that the results reflect change greater than random variation. Appropriate statistical analysis is important: by-item statistical analysis methods are strongly recommended and a methodology is presented using WEighted STatistics (WEST).The last decade has seen a rise in the influence of evidence-based practice (EBP) approaches to both service delivery decisions and the practice of therapy. This can be seen in the development of many sets of EBP guidelines and the teaching of evidence-based principles being adopted as a core element of many training courses for clinicians. We are wholly supportive of this. Central to EBP is the issue of what constitutes good evidence.Some EBP practitioners have asserted that randomised controlled trials (RCTs) are almost always the best evidence. A number of people have argued that, for behavioural therapies with heterogeneous populations, RCTs may not often yield useful data. While RCTs may indicate that therapy should be offered to individuals with language impairment, they do not, at least in their current form, provide answers to the "how" of therapy. They do not generally allow us to relate outcome to individual profiles and therefore cannot answer questions about which specific therapy approach is most likely to help a given individual.
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