*2.1. Study Design*

A cohort study was performed in two waves. The first wave of cross-sectional study was conducted in spring 2020 and the second wave in autumn 2020, at a large public technical university in the south of Poland. The necessary sample size was determined using G\*Power ver. 3.1.9.4. Software [76,77]. The sample size equaled 167 people, if calculated a priori for bivariate correlation with *r* = 0.30, *p* < 0.01, and 95% CI, and equaled 108 people for a linear multiple regression model with two independent variables, effect size *f* <sup>2</sup> = 0.15, *p* < 0.01, and 95% CI. University students were recruited through the online e-learning platform at a university. The invitation to participate in the study (with a link to the on-line survey) was provided on the Moodle platform from 3 March to 29 April 2020, during the first wave (W1) of the COVID-19 pandemic. The information about the study was provided and informed consent was obtained using the first page of the online questionnaire. Students were informed that participation was voluntary and they could refuse from the survey at any time. No form of compensation was offered as an incentive to participate. The average time for data collection was 20 min. The student sample was highly diverse to minimize sources of bias due to their key characteristics: field of study and study cycle. Among the university students, 986 people responded to the invitation during W1, but 12 refused to participate, and 60 presented more than 5% missing data, so they were excluded from further statistical analyses. Altogether, 914 students participated at measurement time one (W1) and completed all measures, including life satisfaction, perceived stress, and coping with stress (Figure 1).

**Figure 1.** Flow chart of the study sample by recruitment strategy during each wave of the COVID-19 pandemic.

The same procedure was used during the second wave (W2) of the COVID-19 pandemic. The research was conducted from 3 November to 3 December 2020. Among university students, 1354 responded to the invitation. However, 62 refused to participate during W2, and 18 were excluded because more than 5% of their data was missing. Among the 1274 university students during W2, 231 participants matched the following demographic characteristics from W1: birth date, sex, place of residence, faculty, level, and year of the study (Figure 1). Therefore, 231 university students were included in the total sample that was examined using all statistical tests.

The Research Ethics Committee approved the study protocol at the University of Opole, Poland (1/2020). The study followed the ethical requirements of anonymity and voluntariness of participation. Following the Declaration of Helsinki, written informed consent was obtained from each student before inclusion. We received no specific funding for this work. This study is part of an international research project, "Well-being of undergraduates during the COVID-19 pandemic: International study", registered at the Center for Open Science (OSF) [78].
