Chapter 6 104 Pre-existing knowledge A previous performed systematic review and meta-analysis of prognostic factors of adherence to home-based exercise therapy in patients with chronic diseases [19] formed the basis. According to this systematic review and meta-analysis higher exercise adherence was predicted by self-efficacy, exercise history, motivation, education, physical health, comorbidities, depression, fatigue, and support from a healthcare provider [19]. Further, in paramedical professions it is widely accepted that the treatment regimen alone cannot fully account for patient outcome [20]. The relationship between patient and therapist (therapeutic alliance) has been viewed as an important determinant of treatment outcome and is considered central to the therapeutic process [21]. Fundamental mechanism To model the fundamental psychological determination of behavior, the theory of planned behavior (TPB) was used. TPB supposes that a person’s intention to perform a behavior is the major determinant of that behavior [22]. Furthermore, a person’s intention is determined by three theoretically independent variables: a person’s attitude, subjective norm, and perceived behavioral control (PBC) [22]. Other criteria for selection The process of prespecifying predictors takes into account the prevalence (between 20% and 80%) of the candidate predictors in the dataset. Further, it has been taken into account that prediction models tend to include predictors that are quite readily available, not too costly to obtain, and can be measured with reasonable precision [23]. To achieve parsimony, and reliability of predictions that are provided by the prediction model, eight candidate predictors could be chosen. The minimum events per variable for obtaining good predictions may be ten [23]. The smallest event category must be assumed (limiting sample size); this was the event ‘adherent’ and included 84 cases. With an EPV rate of at least ten, up to eight predictors can be simultaneously included in the full model for reliable results. For the feasibility of the prediction model, we chose to categorize candidate predictors into five domains: demographic data, disease characteristic data, planned behavior constructs, psychological constructs, and exercise and fitness variables. Based on the results of the systematic review (high and moderate quality evidence), TPB, and the other criteria for selection, the following candidate predictors were included: planned behavior constructs: PBC, attitude, subjective norm, intention; exercise and fitness variables: exercise history; disease characteristics: depression, Medical Research Council dyspnea scale (MRC)-score; and psychological constructs; therapeutic alliance (patient-provider relationship).
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