2.4. Participants and data collection

In the present study, the maximum level of attributes, the number of choice sets, and the number of alternatives in each choice set were 3, 8, and 2, respectively. Based on Orme’s rule of thumb formula,²³ at least 94 participants (500 × 3 ÷ 8 ÷ 2) were required to accomplish the study. Participants were enrolled from three representative cities of Shaanxi province, including Yan’an (northern region), Xi’an (central region), and Han Zhong (southern region). These cities represent a distinct socioeconomic level in Shaanxi province,^12, and Xi’an has the highest gross domestic product per capita, whereas Han Zhong has the lowest. Four parks located in the east, west, south, and north of each city were randomly selected, where random intercept access was used to seek the elderly who were willing to participate in the survey. The elderly who had poor recognition or refused to give consent were excluded from the study.

Medical students from the Health Science Center of Xi’an Jiaotong University were recruited as investigators for the study. They were trained adequately about the study design and communication skills. The formal survey was conducted in January 2020. All participants were informed about the study’s aim and relevant knowledge of influenza infection and its consequences. Investigators explained each attribute’s meaning to the participants and helped them complete it. Furthermore, all queries of the participants were addressed by data collectors.