This commentary is on the original article by Geerdink et al. on pages 580-586 of this issue.As studies that examine upper limb function in children with cerebral palsy (CP) proliferate, so have classification systems that describe the capabilities of individuals with CP as well as tools that measure performance of hand and arm use. Researchers and clinicians alike seek reliable and valid measures to make recommendations for practice and to develop questions for future research. Not so long ago, there were few valid and reliable measures that defined the type of impairment or the capacity of the involved upper extremities in children with CP. Thus, making the examination of upper limb use and subsequent design of interventions that might improve hand and arm function not as effective as they could be. However, recent reviews 1 conclude that more valid and reliable tools are available that provide evaluation of upper limb performance, function, and participation for children with CP.The aim of the paper by Geerdink et al. was to develop a validated instrument to evaluate maximal capacity of the more affected hand in bimanual activities. The authors recognize the importance of measuring treatment effects of upper limb performance, as well as measuring the ability to use the limb in optimal conditions in a standardized environment, i.e. capacity.2 In a brief review in this paper of existing valid measures of upper limb function, the limitations of the more frequently used assessments all suggested that none captured the functional capacity of the affected hand, especially while performing bimanual activities. Interestingly, the authors refined a reliable and valid evaluation of upper extremity function, the Modified House Classification (MHC) 3 in order to develop a valid tool that measures the manual capacity of children with unilateral CP. The refined measure defined a new uni-dimensional 14-item set from the MHC indicating that the 14 items worked well together to measure an upper extremity construct. In addition, the new model correlated well with two validated measures of arm and hand function: the Manual Abilities Classification System (MACS) 4 and Abilhand-Kids. 5 The authors of this paper recognized a gap in the evaluation of upper limb function for children with unilateral CP. Specifically, the authors addressed the lack of a valid and reliable measure to examine functional capacity of the involved hand and arm in children with unilateral CP.As more studies are designed to examine the efficacy of therapeutic interventions, extensive descriptions of arm and hand function need to be obtained. Rather than 're-inventing the wheel', appropriate statistical analysis was used to refine an existing measurement tool that can provide a more extensive evaluation of upper limb function in children with unilateral CP.