General discussion 167 diverse behaviors that contribute to clinic-based adherence, has been described to measure adherence in a rehabilitation setting: the Rehabilitation Adherence Measure for Athletic Training (RAdMAT) [6]. However, no Dutch version of this English measurement instrument was available, nor had it been investigated whether this instrument could be used in patients with chronic diseases. In a first step to develop a Dutch measurement instrument to objectively measure clinic-based exercise adherence, a Dutch translation of the RAdMAT, the RAdMATNL, was made in Chapter 2. Further, the inter-rater reliability of the RAdMAT-NL was determined in patients who were undertaking physiotherapeutic rehabilitation in a primary physiotherapy practice. This cross-sectional study showed that the inter-rater reliability was excellent (ICC = 0.98) in this cohort of patients. A measurement instrument must be reliable first before it has a chance of being valid [7]. Since the inter-rater reliability was excellent the structural and convergent validity of the RAdMAT-NL could be determined. In chapter 3, the next step was taken in the development of a valid and reliable measurement instrument. The dimensionality (structural validity) and construct validity of the RAdMAT-NL were explored in patients with chronic obstructive pulmonary disease (COPD) following pulmonary rehabilitation (PR) in a primary physiotherapy practice. In addition, it was examined if the RAdMAT-NL could be used as a single measure of adherence. This prospective study revealed that the RAdMAT-NL had good structural validity; principal axis factoring demonstrated two dimensions (Participation and Communication) of the RAdMAT-NL explaining 50.8% of the total variance, and good internal consistency reliability; McDonald’s omega was 0.92 and 0.68 for the Participation and Communication subscales, respectively. The construct validity was also good; medium to large significant positive correlations (0.37-0.78) between the RAdMAT-NL subscale Participation and different measures of adherence were demonstrated. Rasch analysis demonstrated that without the communication items (Andersen LR-test, p-value < 0.001) the RAdMAT-NL can be used as a single score for adherence. Since the RAdMAT-NL proved to be a valid and reliable measurement in this cohort of COPD-patients, the RAdMAT-NL was used to measure the outcome variable exercise adherence in the development of the prediction model. The question arose, however, whether adherence is constant over time or does it rather increase or decrease or fluctuate? Since adherence was measured at four measurement moments, it was important to know which of these four measurement moments to use when developing the prediction model. For example, if adherence was stable only in the first three months, the three-month measurement should be used. The prediction model could then only make predictions about adherence in the first three months. In chapter 5, the course of exercise adherence over a 12-month period in patients with COPD receiving prolonged rehabilitation was described. About 70% of patients with COPD in the study population had PR for a year or
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