title: Reliability and equivalence of alternate forms for the Symbol Digit Modalities Test: implications for multiple sclerosis clinical trials.
abstract: BACKGROUND
  Cognitive impairment is common in multiple sclerosis (MS), but is seldom assessed in clinical trials investigating the effects of disease-modifying therapies. The Symbol Digit Modalities Test (SDMT) is a particularly promising tool due to its sensitivity and robust correlation with brain magnetic resonance imaging (MRI) and vocational disability. Unfortunately, there are no validated alternate SDMT forms, which are needed to mitigate practice effects.
  
  
  OBJECTIVE
  The aim of the study was to assess the reliability and equivalence of SDMT alternate forms.
  
  
  METHODS
  Twenty-five healthy participants completed each of five alternate versions of the SDMT - the standard form, two versions from the Rao Brief Repeatable Battery, and two forms specifically designed for this study. Order effects were controlled using a Latin-square research design.
  
  
  RESULTS
  All five versions of the SDMT produced mean values within 3 raw score points of one another. Three forms were very consistent, and not different by conservative statistical tests. The SDMT test-retest reliability using these forms was good to excellent, with all r values exceeding 0.80.
  
  
  CONCLUSIONS
  For the first time, we find good evidence that at least three alternate versions of the SDMT are of equivalent difficulty in healthy adults. The forms are reliable, and can be implemented in clinical trials emphasizing cognitive outcomes.
authors.affiliation: SUNY Buffalo School of Medicine, Neurology, Buffalo General Hospital, Buffalo, NY 14203, USA. benedict@buffalo.edu
authors.forename: Ralph H B
authors.initials: RH
authors.lastname: Benedict
authors.forename: Audrey
authors.initials: A
authors.lastname: Smerbeck
authors.forename: Rajavi
authors.initials: R
authors.lastname: Parikh
authors.forename: Jonathan
authors.initials: J
authors.lastname: Rodgers
authors.forename: Diego
authors.initials: D
authors.lastname: Cadavid
authors.forename: David
authors.initials: D
authors.lastname: Erlanger
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D003071:Cognition; D003072:Cognition Disorders / Q000175:diagnosis* / Q000209:etiology / Q000523:psychology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000150:complications / Q000175:diagnosis* / Q000523:psychology; D009483:Neuropsychological Tests; D009518:New York; D011237:Predictive Value of Tests; D011594:Psychometrics; D015203:Reproducibility of Results; D012107:Research Design