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The case for single-case studies in treatment research—comments on Howard, Best and Nickels “Optimising the design of intervention studies: critiques and ways forward”

Howard, Best, and Nickels (2014) provide an informative state-of-the-art summary of the evolution of single-case treatment research design and some welcome suggestions for designs and statistics to increase the validity of this approach to treatment research. For many aphasiologists, the need for and value of the single-case design is well appreciated, but is challenged by the need for statistical methods and design that allow attribution of language improvement to the treatment. In step with previous advances in design of single-case study research (e.g., Fisher, Kelley, & Lomas, 2003; Howard, 1986; McReynolds & Kearns, 1983; Robey, Schultz, Crawford, & Sinner, 1999; Thompson, 2006, for review), Howard et al. offer a case for considering some methodological adjustments to our current approach to singlecase designs for treatment research that could improve their validity and in so doing bolster the acceptance of this approach as an alternative to the medical model of treatment validation. Historically, patient-oriented research in the communication sciences has faced numerous challenges to its acceptance by mainstream cognitive psychology as a valid approach to the study of the cognitive organisation of language. Levelt, Roelofs, and Meyer (1999), for example, suggested that although it is appropriate to use experimental paradigms developed to study normal language processing for studies of aphasia, we should not expect behaviour of a damaged system to conform to a theory of normal language. This cautionary note sidestepped the challenge of developing a model of language processing that could account for both normal and impaired language behaviours. In fact, cognitive neuropsychological research has played an important role in testing the validity of models of normal language processing by examining their ability to account for impaired language (e.g., Dell, Schwartz, Martin, Saffran, & Gagnon, 1997; Howard & Franklin, 1988; Patterson & Shewell, 1987; Rapp & Goldrick, 2000). Levelt et al. (1999) noted further that a contribution of neuropsychology should be to identify explicit component processes of word production. It is, in fact, this line of research in cognitive neuropsychology, identification of associations and dissociations of components and processes of the language system, which has informed our models of diagnosis and treatment of aphasia used in research and clinical practice. Impairment-based treatments of aphasia have evolved from a focus on communication task-level training (naming, comprehension, repetition) to treatments based on more nuanced descriptions of language impairment that include type of linguistic representations affected (e.g., semantic, phonological, syntactic), processing components affected (access, retrieval, short-term maintenance) and involvement of other cognitive processes (e.g., working memory, executive functioning). Although language impairment profiles are unique to the individual, impairment-based