*4.3. Limitations of the Study*

There are certain limitations to this study. First, the online recruitment method has several limitations. As the research was performed during the lockdown related to the COVID-19 pandemic, all students used remote web-based technologies to learn in virtual educational platforms. An online survey was the only way to perform research on student well-being during the general quarantine. However, the invitation was distributed via Facebook groups, Viber groups, and Telegram channels, so students who do not use these websites could not respond. The online survey results may not be consistent with "paper and pencil" questionnaires. Although the study sample was substantial, there were more female respondents. This proportion is consistent with sex prevalence in typical universities (although technical universities comprise more males). Further research may include a more balanced proportion of the sexes in the study sample. As this research was performed in universities exclusively, the results of this study may not be generalized into more technically focused types of universities. Moreover, this study's results cannot be generalized into other adult populations, since participants in this study were exclusively university students. Some demographic variables, such as ethnicity, income, employment, marital status, or household members, were not included in the questionnaire. Thus, we do not know if these variables would be more useful as covariates in analyzing the relationship between PA and mental disorder. Further research should include more demographic variables related to the mental health of university students. We used a simple, dichotomous division of people into active and inactive groups according to the WHO recommendation of PA level over the past week. However, other survey questions concerning more details of the level of PA (mild, moderate, vigorous) or time of each of the types could be used in future research. Finally, due to the cross-sectional character of research, the results of the regression analysis should be treated with caution.
