Chapter 6 116 and Expectancy Questionnaire (CEQ) for expectancy and credibility (Devilly & Borkovec, 2000) to strengthen the validity of our measurements. Finally, to better understand the factors influencing changes in feelings of certainty and guidance, we explored associations between age, education level, length of waiting period, baseline level of CGQ, and the change in CGQ scores in both the intervention and control group. 6.2.3.5 Qualitative insights In line with our mixed-methods approach, we complemented the quantitative data for adherence, acceptance, and feelings of certainty and guidance with qualitative insights collected with semi-structured interviews. We asked questions relating to reasons for adherence (e.g., Why did or did you not succeed in using the intervention daily?), acceptance (e.g., What was your experience with using the intervention?) and effects on feelings of certainty and guidance (e.g., How has the intervention been able to help you the most during the waiting period?). See Supplementary Appendix 2 for the full interview guide. 6.2.4 Procedures We performed assessments in both study groups during two contact moments: initially upon enrollment in CR (T1) and again just before the CR program began, usually 2-8 weeks after T1 (T2) (See figure 1). At T1, participants were briefed on the study, signed consent forms and completed demographic and CGQ questionnaires. The intervention group received additional information about the smartphone app and help installing it. At T2, both groups completed a second CGQ questionnaire, with the intervention group also submitting usage data, filling out an acceptance questionnaire and participating in a phone interview. Participants received a 20-euro gift voucher for their participation. 6.2.5 Intervention The CapriXpress application is a tailored digital intervention developed to support patients with a low SEP during their waiting period between discharge from the hospital and the start of their CR. This intervention was co-designed in a participatory design study with the target group (Faber et al., 2023a). The CapriXpress application addresses the need for certainty and guidance for people with a low SEP during their waiting period. To enhance patient adherence and acceptance of the intervention, we ensured that the intervention is grounded in established theoretical frameworks, namely the Taxonomy of Behavior Change Techniques (BCT) (Michie et al., 2013) and the Persuasive Systems Design model (PSD) (Oinas-Kukkonen & Harjumaa, 2009) (see Table 1). Combining the two frameworks is valuable as it combines the rigor of
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