Plink 1.07 (Purcell, Neale, Todd-Brown, Thomas, Ferreira, Bender, Maller, Sklar, de Bakker, Daly & Sham, 2007) was used to verify genetic data quality and test for departure from Hardy-Weinberg equilibrium. Pairwise linkage disequilibrium was evaluated with Haploview 4.1 (Barrett, Fry, Maller & Daly, 2005) and expressed as D′. Hierarchical linear regression conducted using SPSS 17 (Chicago, IL) was used to test whether specific dopamine SNPs moderated the relations between working memory change over time (i.e. independent variable) and improvements in ADHD symptoms (i.e. dependent variable). A statistical moderator affects the direction and/or strength of the independent variables relation on the dependent variable. Here, moderation would be supported by a significant SNP × digit span forward/backward interaction. It is important to note that the main effect significance of the independent and moderator variables is not directly relevant conceptually to testing the moderator hypothesis; can be misleading depending on how variables are coded; and therefore should not be interpreted (Baron & Kenny, 1986). Model-based plotting of the simple slopes showing the relationship between change in working memory and change in ADHD symptomatology
