A standardized neuropsychological test battery was administered to all subjects. Tests were administered in English (U.S. sites) or Afrikaans (South African site). The tests included in this battery were: the Cambridge Neuropsychological Test Automated Battery (CANTAB: Delayed Matching to Sample, Intra-Extra Dimensional Shift, Choice Reaction Time, Simple Reaction Time, Spatial Working Memory subtests) (Cambridge Cognition, 2006) and the Delis-Kaplan Executive Function System (D-KEFS: Color-word Interference, Trail Making, 20 Questions, Tower, Verbal Fluency, Design Fluency subtests) (Delis et al., 2001). Full Scale IQ (FSIQ) scores were obtained using the Wechsler Intelligence Scale for Children – fourth edition (WISC-IV; Wechsler, 2004). Test results were administered and scored by the examiner according to published test manuals and rechecked by a second trained person. Scores were converted to standard scores or z-scores according to age norms when available and data were entered into a centralized database, which required double entry for verification of accuracy. Given the large number of variables available from this dataset, we selected a parsimonious set of summary variables from each measure. Variables were selected using clinical judgment and expertise of
