title: Single-case research in neuropsychology: a comparison of five forms of t-test for comparing a case to controls.
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).
authors.affiliation: School of Psychology, University of Aberdeen, UK. j.crawford@abdn.ac.uk
authors.forename: John R
authors.initials: JR
authors.lastname: Crawford
authors.forename: Paul H
authors.initials: PH
authors.lastname: Garthwaite
mesh_terms: D000704:Analysis of Variance; D008959:Models, Neurological; D009010:Monte Carlo Method; D009484:Neuropsychology / Q000379:methods*; D012107:Research Design; D013223:Statistics as Topic