Anne Fleur Kortekaas-Rijlaarsdam

CHAPTER 2 38 None of our mediators or moderators influenced MPH effects on math and reading accuracy or productivity. This may be because most variance is due to random error as indicated by the low I 2 values obtained in our meta-analyses (Borenstein et al., 2009). However, as there is large uncertainty (large confidence intervals) in heterogeneity estimates such as I 2 , we deemed our meta-regression relevant (Ioannidis, Patsopoulos, & Evangelou, 2007). In particular, we hypothesized that teacher-rated symptom improvements would mediate MPH-effects on academic performance, but the results from our meta-analysis did not confirm this hypothesis for MPH-effects on math accuracy. Unfortunately, the number of studies reporting on teacher-rated symptom improvements and math productivity as well as reading was insufficient for meta- regression. Further, most studies reporting on on-task behavior and academic performance measures, indicate simultaneous improvements on both, with improvements in on-task behavior ranging from 2.9% to 12.0% ( Barkley, Fischer, Newby, & Breen, 1988; Carlson, Pelham Jr, Milich, & Dixon, 1992; Cunningham, Siegel, & Offord, 1991; Froehlich et al., 2014; Gorman, Klorman, Thatcher, 2006; Pelham et al., 2001; Pelham et al., 1985; William E Pelham, Vodde-Hamilton, Murphy, Greenstein, & Vallano, 1991; Pelham Jr. et al., 1987; Pelham Jr., Walker, Sturges, & Hoza, 1989; Pelham Jr et al., 1990). Unfortunately, the number of studies in the current study was too small to test the mediating effects of on-task behavior using meta-regression. Taken together, our results do not support a mediating role for classroom expressed ADHD symptoms in the relationship between MPH and math accuracy, but this may be different for productivity measures as behavioral improvements in the classroom are generally seen as a prerequisite for academic improvements, especially academic productivity. Furthermore, cognitive improvements may be more relevant here than symptom improvements, as deficits of children with ADHD are apparent for those cognitive functions that are especially important for academic performance, e.g. attention, working memory and response inhibition (Biederman et al., 2004; Mayes & Calhoun, 2007b; Preston et al., 2009; Thorell, 2007). Possibly MPH-related improvements in cognition play a large role in academic improvement, compared to behavioral improvements – and these act through different pathways than those driving symptoms. This is consistent with a recent study by Coghill et al. (2014) showing that while MPH improves both symptoms and some aspects of cognition these effects seem to be independent. Demographic and disorder related variables included in our analysis (age, gender, ADHD subtype and ADHD severity) did not moderate MPH efficacy on math performance either. The absence of a moderating effect of age and comorbid disorders is in line with the results of a recent meta-analysis on behavioral improvements with