Development and validation of an exercise adherence prediction model 103 Patients who were willing to participate were contacted by the researcher per email for further information on their participation, and to obtain informed consent. For patients without an e-mail address, this was done by their physiotherapist during their visit to the physiotherapy practice. Patients were also recruited via the COPD patient organization in the Netherlands and Belgium who, in turn, invited their physiotherapist for participation (no financial compensation was offered to the physiotherapists). Procedures After signing informed consent and inclusion, patients completed an online (Qualtrics) or paper form on their demographic characteristics: age (years), gender (male/female), country (the Netherlands/Belgium), education (low/middle/higher), smoking status (never smoked/quit smoking/still smoking) and medication adherence (yes/no). About simultaneously, the physiotherapist provided information on the characteristics of the disease of the patient: classification of severity of airflow limitation (GOLD classification) (GOLD II/III/IV), degree of baseline functional disability due to dyspnea (MRC-score) (0/1/2/3/4/5), duration of COPD since diagnosis (years), and duration of physiotherapy (years). Both patients and physiotherapists provided information at baseline and 12 months after inclusion. Patients provided additional information three months after inclusion. Data collection All questionnaires for both patients and physiotherapists were prepared in Qualtrics (online survey software) [18]. The questionnaires were sent by the researcher from Qualtrics to the participants at the different measurement moments. Answers to the questionnaires were automatically collected in Qualtrics. If a questionnaire was not completed after two weeks, an automatic reminder was sent. The researcher entered the responses of patients who completed a paper questionnaire into Qualtrics to obtain a complete data file of all participants. Prior to data analysis, the raw data file was checked by the researcher for any input errors. If input errors were found, the researcher contacted the respective participant (patient and/or physiotherapist), if possible. In this way, the raw data file was made technically correct and consistent. Data were anonymized by deleting the e-mail addresses and adding a PID number. Based on the PID number measurements of all measurement moments could be merged. Candidate predictors To construct a prespecified model (to limit selection bias and overfitting), candidate predictors were identified through: pre-existing knowledge, fundamental mechanisms, and specified criteria for selection.
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