Development and validation of an exercise adherence prediction model 117 model to limit selection bias and overfitting. A recent performed systematic review indicated variables associated with adherence [19]. Further, TPB as the state-ofthe-art model to predict adherence behaviors in patients with a chronic disease, was used [36]. This study also has some limitations. Because we wanted a simple model, and because the sample size was not large enough, we accepted uncertainty about the assumptions; we chose to consider the continuous data as linear (cubic splines would cost too many degrees of freedom, making the model overfit). In terms of handling continuous predictors, researchers strongly advise against converting continuous variables into categorical variables, due to information loss and reduced predictive accuracy [31]. That is why we choose to accept the uncertainty about the assumptions. Therefore, future validation studies in other cohorts should be performed. In a previous study [37], we demonstrated that in this cohort of COPD patients adherence was constant over 12 months. Therefore, this prediction model can only be considered valid over this time period. Thereby, the purpose of this prediction model was to better support the current population of COPD patients. Follow-up research should examine whether this prediction model (without alliance) is applicable to patients starting PR, and if this model is valid over a longer period than 12 months. Furthermore, future research might examine whether a reduction in depressive symptoms, an improvement in alliance, intention, and MRC-score actually lead to better exercise adherence in both patients who have had PR for some time and those who are at the beginning of their PR. Physiotherapy practices were offered a financial compensation if they participated in the full study. The compensation was offered at the end of the study as a token of gratitude and appreciation [38]. This had no impact on the study recruitment as it was given at the end of the study. The financial compensation was too low to influence physiotherapist retention and therefore did not serve as an incentive to prevent a physiotherapist from dropping out of the study. Clinical implications Adherence is important in many aspects of healthcare as it is related to clinical outcomes, and to the (economic) burden for healthcare providers [39]. To keep healthcare affordable and improving patient outcomes, attention must be paid to adherence [40]. Information from this prediction model can be used by healthcare providers to facilitate discussions regarding clinical care and target services to better manage COPD and make more efficient use of health care by patients receiving prolonged (70% of patients in this cohort followed PR for ³ 1 year) pulmonary rehabilitation in a primary physiotherapy practice. Counselling can possibly focus on patients who need it the most, the ones who are non-adherent. Patients who are adherent require less counselling; their self-management ensures
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