In addition to increasing power, using symptoms or rating scales as a basis for phenotype selection had the advantage that hyperactive–impulsive and inattentive symptoms could be assessed independently in the combined subtype ADHD of the GAIN/IMAGE participants. However, a prerequisite for this quantitative approach to be valid is that ADHD is a disorder at the extreme of a continuum observed in the population. Evidence suggesting that this is indeed true comes from twin studies showing strong heritability of quantitative measures of ADHD (Lasky-Su et al. 2008b, and references herein) as well as from a proband–sibling comparison in IMAGE (Chen et al. 2008; Thapar et al. 2006). Starting from the cleaned GAIN dataset, Lasky-Su restricted the analysis to 429,784 autosomal SNPs (as FBAT cannot handle X-linked markers) and 909 complete families. 87% of the probands were male, and the mean age of probands was 10.88 years. The average total number of symptoms was 16.11, the number of hyperactive–impulsive symptoms was 8.11 and the number of inattentive ones was 7.98.
