title: Intra-subject variability in attention-deficit hyperactivity disorder.
abstract: BACKGROUND
  This study is based on a comprehensive survey of the neuropsychological attention-deficit hyperactivity disorder (ADHD) literature and presents the first psychometric analyses of different parameters of intra-subject variability (ISV) in patients with ADHD compared to healthy controls, using the Continuous Performance Test, a Go-NoGo task, a Stop Signal Task, as well as N-back tasks.
  
  
  METHODS
  Data of 57 patients with ADHD and 53 age- and gender-matched controls were available for statistical analysis. Different parameters were used to describe central tendency (arithmetic mean, median), dispersion (standard deviation, coefficient of variation, consecutive variance), and shape (skewness, excess) of reaction time distributions, as well as errors (commissions and omissions).
  
  
  RESULTS
  Group comparisons revealed by far the strongest effect sizes for measures of dispersion, followed by measures of central tendency, and by commission errors. Statistical control of ISV reduced group differences in the other measures substantially. One (patients) or two (controls) principal components explained up to 67% of the inter-individual differences in intra-individual variability.
  
  
  CONCLUSIONS
  Results suggest that, across a variety of neuropsychological tests, measures of ISV contribute best to group discrimination, with limited incremental validity of measures of central tendency and errors. Furthermore, increased ISV might be a unitary construct in ADHD.
authors.affiliation: School of Psychology, University of Wales, Penrallt Road, Bangor, Gwynedd, Wales, UK. c.klein@bangor.ac.uk
authors.forename: Christoph
authors.initials: C
authors.lastname: Klein
authors.forename: Katharina
authors.initials: K
authors.lastname: Wendling
authors.forename: Paul
authors.initials: P
authors.lastname: Huettner
authors.forename: Hans
authors.initials: H
authors.lastname: Ruder
authors.forename: Martin
authors.initials: M
authors.lastname: Peper
mesh_terms: D000293:Adolescent; D000704:Analysis of Variance; D001289:Attention Deficit Disorder with Hyperactivity / Q000503:physiopathology* / Q000523:psychology*; D016022:Case-Control Studies; D002648:Child; D005260:Female; D006801:Humans; D007360:Intelligence; D007389:Internal-External Control; D008297:Male; D009483:Neuropsychological Tests / Q000706:statistics & numerical data*; D025341:Principal Component Analysis; D011594:Psychometrics; D011930:Reaction Time / Q000502:physiology