Making sense of deviance: identifying dissociating cases within the case series approach.

The case series approach in cognitive neuropsychology provides a means to test theories that make quantitative predictions about associations between different components of the cognitive system [Schwartz, M. F., & Dell, G. S. (2010). Case series investigations in cognitive neuropsychology. Cognitive Neuropsychology, 27, 477-494]. However, even when the predicted association is borne out the study may include outliers-observations that deviate significantly from the rest of the data. These outliers may reveal individual cases whose cognitive impairments dissociate from other cases included in the study. These dissociating cases can pose a significant challenge to the theory being tested. Using a recent case series that investigated the underlying causes of letter perseveration in spelling [Fischer-Baum, S., & Rapp, B. (2012). Underlying cause(s) of letter perseveration errors. Neuropsychologia, 50, 305-318], I discuss statistical and theoretical issues that arise when using outlier detection techniques to identify dissociating cases in a case series study.