Anne Fleur Kortekaas-Rijlaarsdam

69 MPH EFFECTS ON MATH PERFORMANCE: INFLUENCE OF BEHAVIOR 3 compared to the mean SAT productivity of their TD peers. With medication, children with ADHD improved, although performance still lagged behind by .45 standard deviation compared to the mean SAT productivity of their TD peers. For mathematical word problem accuracy, the difference between children with ADHD taking methylphenidate and TD children was not significant ( t (123) = .62, p = .534). Mediator and Moderator Analysis For those variables showing significant effects of medication, SAT productivity and mathematical word problem accuracy, we used GLMM to test whether improvements in academic performance related to reductions in parent- and teacher-rated ADHD symptoms. When taking medication into account, symptom improvement according to parents was a significant predictor of SAT productivity ( t (67.1) = 2.20, β = .06, p = .031), but did not predict accuracy on mathematical word problems ( t (99) = .86, β = .13, p = .392). The effect of medication on SAT productivity became marginally significant ( p = .057) when symptom improvement was added to the model. When taking medication into account, symptom improvement according to teachers did not predict SAT productivity, nor accuracy on mathematical word problems, respectively ( t (71.6) = 1.29, β < .04, p = .201) and ( t (106.6) = .45, β = .06, p = .658). Next, we performed multilevel mediation analysis to test whether parent-rated symptom reductions mediated the effects of methylphenidate on SAT productivity. The results showed that symptom improvement according to parents was a significant mediator of the effects of methylphenidate on SAT productivity: For β a (effect of methylphenidate on symptom improvement) = 14.11 (SE = 2.39) and β b (effect of symptom improvement on mathematical productivity on SAT) = .06 (SE = .03), the product term β a β b was greater than zero: .79 (SE = .39) with a 95% CI of [.08, 1.60]. For SAT productivity and mathematical word problem accuracy, we explored the influence of the potential moderators: learning ability and ADHD symptom severity. To this end, interaction effects between moderators and medication were calculated. Significant interactions are depicted in Figure 3.2. For SAT productivity, mathematical ability (as measured by SAT performance during the placebo condition) moderated the effects of medication, although the interaction just escaped conventional levels of significance ( χ 2 (1) = 3.21, p = .073). To investigate the nature of this effect, post-hoc analysis were conducted comparing high and low ability groups which were created based on the mean z-score (mean of the TD group, z = 0). Post-hoc analysis showed that medication was only effective in improving mathematical productivity (SAT) for children with below average mathematical abilities