Sonja Mensch

96 Chapter 5 care centres were included. Each of 37 experienced therapists working in the centres, all of whom had more than 10 years experience with the target population, selected one or two of their own clients. Informed consent by parents or legal representatives was obtained from all participants. Study design This was a prospective cohort study with a follow-up of 18 months. Measurements were performed at baseline and up to 5 times during follow-up at 3-month intervals (labelled T1 to T5), representing the standard evaluation period in the day-care centres. Validity testing Construct validity of an instrument is the degree to which the scores of such an instrument are consistent with hypotheses regarding the association between the target instrument and other instruments measuring the same construct (Terwee et al., 2012; Mokkink et al., 2010). Because no instruments exist to measure motor abilities in childrenwith SMD, construct validity of theMovakic was tested against expert judgment of children’s motor ability. Responsiveness is the ability of an instrument to detect change over time of the construct of interest (Terwee et al., 2012; Mokkink et al., 2010). In an ideal situation, we would investigate responsiveness against the effect of a single intervention without confounding variables such as other events or interventions. However, because of the complex study population receiving different interventions, both medical and physiotherapeutic, responsiveness was testedmaking use of the fact that some children would experience meaningful events influencing motor abilities while others would not. For this study, the underlying supposition was that we do not expect changes in motor abilities in a time frame of three months, unless events occurred. The possible occurring events were defined based on the experience of the participating therapists. The next events were selected; surgery, increase of contractures, pain, medication change, change in assistive devices, and other, unexpected events. The proportion of positive versus negative changes in scores was inventoried. Instrument Motor abilitiesweremeasuredby theMovakic instrument (Menschet al., 2015c). Movakic is a digital questionnaire system (currently only available in Dutch), to be administered on-line through a secure internet portal. Information about the instrument, the instruments’ structure, the items and an example of an item is provided in appendix A. Motor abilities are distributed over 12 situations.