DCE survey
hypertensive
We performed the sample size calculation for our 6-month effectiveness trial resulting in a required minimum sample size of 322 hypertensive patients, considering a 20% drop-out rate, 80% power, and a 5% significance level to detect a change in our desired outcome. We also estimated the required minimum sample size for our DCE.For sample size calculation, we took note of the “rule of thumb” method of Johnson and Orme for DCEs [Considering the main effects, we would require a minimum of 250 patients. However, we had to use available 122 trial patients who were recruited from trial patients who had replied to the 12-month telephone survey and reconfirmed consent.Each intervention and control patient was randomly assigned to one of the nine versions of the questionnaire. The DCE survey was administered as a trial exit Computer-Assisted Telephone Interview (CATI) structured around a script. CATI had already been used in the trial to collect other patient-reported data. The database was validated prior to analysis. All interviewers were provided prior training to ensure compliance with data collection instructions, General Data Protection Regulation (GDPR), and ethics, and to maximize approach standardization. In each choice set, interviewers were instructed to present each alternative with its full set of attributes, first alternative A, then B, and to repeat it if necessary.The survey was conducted between 5 and 18 February 2020. Feedback from the first interviews was provided to the research team. We planned for up to eight tentative calls to minimize the loss of patients. Calls were spread over various days and hours to maximize success. When patients answered the call, but the time was inconvenient, the call was re-arranged.