6 Design study II: Development of an ambulatory biofeedback app 197 2.5.3 Materials Hardware A total of 5 mobile phones and 5 smartwatches were used in sets. Each set consisted of a Moto G, third-generation smartphone (5-inch screen size and screen resolution 1280×720 pixels) and a Moto 360, second-generation smartwatch (1.37-inch screen size and screen resolution 360×325 pixels). Both ran a version of Android OS: Android 6.x (KitKat) on the mobile phone and Android Wear 1.5 on the smartwatch. Each set was connected via Bluetooth (and optionally via Wi-Fi) through built-in communication software provided by the Android ecosystem. System Usability Scale The SUS is a commonly used questionnaire that quickly and reliably assesses the usability of a product [38-41]. The SUS [40,41] comprised 10 statements that are scored on a 5-point scale, ranging from totally disagree to totally agree. It contains statements such as “I thought the system was easy to use” and “I thought there was too much inconsistency.” The SUS yields an overall score of the system usability ranging between 0 and 100, where higher scores indicate better usability. For interpretation of scores, we used the guideline provided by Bangor et al [39]. Questionnaires A total of 2 short self-constructed questionnaires were administered to the patients during the first cycle of testing. The first questionnaire (26 items) contained questions on the use context; the patient’s level of experience with technology; general UX of the app; UX of the interface on mobile phone and smartwatch; the use of prompts; and additional questions regarding privacy, perceived risks, and missing/desired features. See Multimedia Appendix 2. The second questionnaire (16 items) contained questions on the patient’s general UX with the mobile phone and smartwatch interface, the use of the diary/note keeping function, and questions about the option to share data with therapists in the future. See Multimedia Appendix 3. Interview After each iteration, a semi-structured interview was held with each patient. The interview consisted of 5 open questions. Participants were asked what they liked about the intervention (app plus hardware), what they disliked about it, what their experiences were regarding the measurement of their bodily signals (PCEA), if they would have made other decisions regarding the design of the app and/or choice of the hardware, and if they had any other remarks about what they would like to see improved or altered in a future version.
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