SIR–It is good to see an increasing number of intervention studies for children with developmental coordination disorder (DCD) as well as for attention-deficit–hyperactivity disorder (ADHD). Watemberg et al.1 went to considerable effort to describe the distribution of comorbidity in children with ADHD with and without DCD and have clarified the characteristics of the children randomized to Phase 2, the intervention study, in the subsequent addendum to their report.2 This addendum is an extremely helpful aid for the clinical interpretation of the results, particularly to illustrate that the 2 (14%) children in the intervention group (group A) who failed to change their Movement Assessment Battery (MABC) score did not differ from the profile of children in the untreated group (group B), either by subtype of ADHD or profile of movement difficulties. However, without information regarding additional associated comorbidities, cognitive ability, or socio-economic status of the children, it is not possible to say whether the two groups differed in a significant way on these key variables owing to chance randomization.
More importantly, however, is the need to consider the possible confounding effect of the treatment group alone having had the benefit of potential practice on the primary outcome measure just before the final outcome testing (4–5wks later). Thus, an alternative conclusion may consider that children with ADHD benefit more from practice effects, indicating the necessity of children with ADHD undertaking a movement assessment test twice within a short period. This would ascertain whether a true movement difficulty is present as opposed to problems in attending to task instructions and/or learning the test items. If this were true it would have important implications for the original calculations of prevalence of DCD in ADHD in Phase 1 of their study and potentially reduce the overall prevalence by half the original figure to 27.6%.
Furthermore, other studies using the MABC to measure movement outcomes have taken into consideration the skew of this impairment scale and also the variations in individual performance by calculating the least detectable difference (LDD) to support interpretation of the significance of the degree of change.3 It is doubtful that the overall results of this study would be different, based upon the total impairment scores on the MABC, calculated from the information provided in the addendum, but it would be helpful for future studies to use the LDD in analyses to consider the differential effects of intervention for children with DCD with or without ADHD.
I am in agreement with the authors that further investigations of therapy interventions, including physiotherapy and/or occupational therapy, to improve the motor performance of children with DCD with and without comorbid conditions are warranted. Nevertheless, this study leaves open the question that the differences in the final MABC scores between the treated and untreated groups may be a testing and methodological artifact rather than progress as a consequence of intervention.