Single subject trials in primary care

From a GP’s perspective, the accompanying article by Janine Janosky on the single subject design is both interesting and stimulating.1 This type of design, as Dr Janosky highlights, is infrequently used in research and has some potential advantages. Most notably, it is the only type of design that can provide information about effects at an individual level. There are obvious benefits in formalising what all GPs do on a day to day basis, namely observing the effects of individual treatments on individual patients. However, the article suggests a scope and potential for the n = 1 trial that I would take issue with, and the author fails to adequately describe the limits and disadvantages of this type of design.

While single subject designs have the potential of examining effects at an individual level, they do not provide data that can readily be applied to others. The author does mention that the generalisability of results from this type of study is limited, but goes on to suggest that if a subject that is “representative of the general type of patients for which this intervention would be used” then the results become more generalisable. A person can be chosen that has a certain disease at a certain stage and with certain sociodemographic characteristics. But is this person really representative? How do we know exactly which variables are relevant to the effect being shown? And how can we judge …