2.2. Study population and sampling
PHYSIOLOGICAL STRESS, DISEASE
Full-time students who were aged 18 years old or above and had the ability to comprehend our English-language questionnaire were recruited from a medium-sized university in Hong Kong. The exclusion criteria included self-reported mental illness, self-reported symptoms of disease, taking medicine, consuming caffeinated drinks and food, and consuming alcohol during the weeks of the study. Fifty-six participants were recruited, three of whom dropped out after registration due to other commitments, leaving an eligible sample of 53. All 53 participants completed the psychological measurements, and 28 participants agreed to the collection of data on the physiological stress variables. Because travel to the on-campus laboratory was required amidst the constraints of the fourth wave of COVID-19 in Hong Kong, we acquired a smaller sample of physiological stress variables than we would have acquired otherwise. The retention rate of the participants by the last week of the intervention was 84.5%. The study protocol was reviewed and approved by the Hong Kong Baptist University's Research Ethics Committee (REC/19-20/0306). All of the participants provided their prior informed consent. Convenience sampling was adopted through print advertisements and social media, including the school email network, Facebook, and Instagram, from December 2020 to November 2021. The participants were instructed to register and complete a demographic survey online. The eligible students who confirmed that they could attend all nine virtual nature experience sessions were then informed of arrangement of the interventions. To encourage retention, an incentive of HK$100 was offered to each participant who completed the study. The study was conducted between January and November 2021, during the fourth wave of COVID-19 pandemic, during which time Hong Kong had 12,436 confirmed cases (