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Smart Data Collection for the Assessment of Treatment Effects in IBS 223 9 without data-loss. Encountered hurdles included difficulties installing the application during the screening visit due to connectivity failure, not receiving reminder notifications, inaccurate visual scaling of questionnaires on smaller smartphone screens, updates of Android or iOS operating systems that interfered with prior processes, and connectivity failure due to server maintenance. All (documented) technical issues and their (short- term) consequences are presented in Table 9.2 . Web-based questionnaires: patients’ adherence and completeness Adherence to the web-based questionnaires was also excellent. One patient did not complete the questionnaires at the end of the treatment period; all others completed all questionnaires until the end of the study or until discontinuation ( N =11 discontinued with the study). Halfway through the study duration, however, a routing error in one questionnaire became apparent. Although this mistake was corrected immediately, the error had already led to missing data for that (one) particular question in 23.3%-54.0% of all patients, depending on measurement moment ( Supplementary Table S9.1 ). No missing items were found in other questionnaire items. Investigators’ adherence to the eCRF Adherence of the investigators to the eCRF was excellent with a completion rate of more than 99%. In total, there were 27 patients with at least one missing variable in the case report file, 11 of whom discontinued with the study during the treatment period (the missing values comprehend follow-up calls that were not conducted). The remaining 17 cases with missing data were because of missed follow-up calls (in 11 cases, one out of three follow-ups was missed), not registering if additional information about the six-months follow-up period was given, not registering the date of the last menstruation, not registering if the GP was informed about participation in the study, or not registering the number of capsules that were reported not to be taken during one of the follow-up calls.