General discussion 171 provided in the future, especially when the measurement is new to the user, as in the study described in chapter 2. Before using the RAdMAT-NL, the raters, the physiotherapists, evaluated three patients together to reach consensus on the use of the RAdMAT-NL and indicating the assessment method was standardized. Another problem may be that the data were collected during the COVID-19 pandemic. The pandemic affected the behavior of patients and physiotherapists. Communication and treatment may have taken place in a different way than it would have outside the pandemic. This was demonstrated by the study by Menting et al [13]. In their research group, one in five patients reported having modified their self-management during the pandemic, both positively and negatively, out of fear of becoming ill [13]. If patients have been more consistently adherent or non-adherent, this may reduce the generalizability of the results. However, these effects are likely to be similar to contextual effects caused by, for example, seasons or changes in treatment reimbursements, which interfere with scientific research focused on practice. The COVID-19 pandemic also resulted in hardly any new patients starting pulmonary rehabilitation during this period. As a result, the study cohort consisted of patients with COPD who had been following prolonged PR. Of the COPD patients included in the study, 70% had already had PR for a year or more. This means that the results of this thesis can be generalized to patients who have been following prolonged PR. When the results are applied to patients starting PR, the results should be interpreted with some caution. Methodological considerations Since data collection took place during the COVID-19 pandemic, it was initially difficult to include enough participants. Therefore, it was decided to extend the study to Belgium. In retrospect, this was a good choice as it increases the generalizability of the results (more diversity helps to ensure the generalizability of the study results [14]). This was supported by the fact that there was no difference in results between patients from the Netherlands and from Belgium (p = 0.23). Also, the participation of patient organizations in the Netherlands and Belgium contributed to the inclusion of participants. Involving patient organizations addresses the desire to involve patients more in research [15] (patients were not only research participant, but they also helped to include other patients and physiotherapists). Possibly because of this collaboration between patients, physiotherapists and researcher, the response rate to the questionnaires was high and few participants left the study early. There is some evidence that patient and public involvement improves the efficiency and value of research via a number of mechanisms: increasing its relevance to patients; improving recruitment and retention rates of research participants; extending the range of people represented in research studies [15]. So possibly this patient involvement minimized loss to follow-up. It has also been demonstrated that motivation ultimately affects study validity [16]. Motivation affects how attentive participants are. The strongest
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