Single case research in neuropsychology: a comparison of five forms of t-test for comparing a case to controls

Crawford, John R. and Garthwaite, Paul H. (2012). Single case research in neuropsychology: a comparison of five forms of t-test for comparing a case to controls. Cortex, 48(8) pp. 1009–1016.

DOI: https://doi.org/10.1016/j.cortex.2011.06.021

Abstract

Five inferential methods employed in single-case studies to compare a case to controls are examined; all of these make use of a t-distribution. It is shown that three of these ostensibly different methods are in fact strictly equivalent and are not fit for purpose; they are associated with grossly inflated Type I errors (these exceed even the error rate obtained when a case’s score is converted to a z score and the latter used as a test statistic). When used as significance tests, the two remaining methods (Crawford and Howell’s method and a prediction interval method first used by Barton and colleagues) are also equivalent and achieve control of the Type I error rate (the two methods do differ however in other important aspects). A number of broader issues also arise from the present findings, namely: (a) they underline the value of accompanying significance test results with the effect size for the difference between a case and controls, (b) they suggest that less care is often taken over statistical methods than over other aspects of single-case studies, and (c) they indicate that some neuropsychologists have a distorted conception of the nature of hypothesis testing in single-case research (it is argued that this may stem from a failure to distinguish between group studies and single-case studies).

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