Participants and procedures
The present study was a three-wave, repeated cross-sectional telephone survey among young adults in Hong Kong between 2021 and 2023. The three assessment waves were held between August and November 2021 annually in 2021 (before the fifth COVID-19 wave), 2022 (after the fifth COVID-19 wave), 2023 (after the end of the pandemic), respectively [12]. The surveys were conducted by an independent survey agency in undertaking various social surveys with expertise in survey implementation and data security. Simple random sampling was used to retrieve 6500 mobile phone numbers from the Office of the Communications Authority in Hong Kong. Trained interviewers contacted potential participants in selected samples from 6:30 pm to 10:30 pm using a computer-assisted telephone interviewing system. Inclusion criteria of the participants were aged 18 - 35, residence in Hong Kong for at least 6 months, and ability to understand Cantonese. Invalid cases with age outside 18-35 years and non-working mobile phone numbers were excluded.
Eligible participants provided oral informed consent before completing a standardized questionnaire in 10-15 minutes. During the phone interviews, on-site validity checks were performed on range and consistency of the collected responses to ensure accuracy and completeness. To ensure proper adherence of the fieldwork procedures, 300 interviews were randomly drawn for quality checking and 91.7% of them confirmed that they participated in the phone interview with consistent answers for four questions in the quality back-check questionnaire as the completed questionnaire. The survey data were weighted with reference to 2021 population census data by the Census and Statistics Department of Hong Kong to adjust proportionally for the age and gender. The authors assert that all procedures contributing to this work comply with the ethical standards of the relevant national and institutional committees on human experimentation and with the Helsinki Declaration of 1975, as revised in 2013. All procedures involving human subjects/patients were approved by the Human Research Ethics Committee of the University of Hong Kong (Reference number: EA210185). Verbal informed consent was obtained from all participants, and they understood that they could withdraw anytime.
The present study recruited samples of 1472, 1238, and 1226 eligible adults in the 2021, 2022, and 2023 telephone surveys. Response rates of the survey, which denoted the ratio of successful interview cases to the sum of successful interview cases, non-contact cases (not answering the call), and refusal cases, were 68.9%, 61.5%, and 62.2% in the three waves of surveys, respectively. Overall, half (49.7%) of the sample were females. The sample had a mean age of 27.0 years (SD = 4.74; range = 18 – 35) and was classified into two age groups of 2312 young adults (58.7%: aged 26 – 35) and 1624 youths (41.3%: aged 18 – 25). The majority of the sample attained tertiary education level (73.8%) and were working (80.7%).
Measures
The standardized questionnaire included demographic characteristics and measures on situational impact, societal pessimism, meaning in life, psychological distress, social withdrawal, and suicidality. The questionnaire was pilot-tested in 20 young adults in July 2021 to refine questionnaire wordings and operational procedures.
Situational impact
Situational impact was measured by the Mental Impact and Distress Scale: COVID-19 [2]. This scale assessed the perceived negative impact of COVID-19 pandemic in five domains over the past four months: study/job, family, social, physical, and financial, on a 5-point Likert scale (1 = “not at all” to 5 = “very severe”). This scale showed acceptable reliability (Mcdonald’s Omega ω = 0.70 – 0.85) across the three waves in the present study.
Depressive symptoms
The 4-item Patient Health Questionnaire [24] was used to assess the level of depressive symptoms of the respondents over the past two weeks. The items were scored on a 4-point Likert scale (0 = “not at all” to 3 = “nearly every day”) and the total PHQ-4 score had a theoretical range from 0 to 12. This scale showed good reliability (ω = 0.77 – 0.88) across the three waves.
Meaning in life
Meaning in life was evaluated by the 3-item Presence subscale of the Meaning in Life Questionnaire - short form [25]. The items were scored on a 7-point Likert scale (1 = “absolutely untrue” to 7 = “absolutely true”) and the total meaning in life score had a theoretical range from 1 to 7. This scale showed good reliability (ω = 0.85 – 0.89) across the three waves.
Societal pessimism
Societal pessimism was evaluated by how much the respondents agree with the following three statements: 1) I am pessimistic about the future of my society; 2) the glory days of my society have passed; and 3) the society I am living in is weakening overall. The items were developed with reference to empirical framework on societal unease by Steenvoorden [26]. The items were answered on a 7-point Likert scale (1 = “strongly disagree” to 7 = “strongly agree”) and the total score of societal pessimism had a theoretical range from 1 to 7. This scale showed acceptable reliability (ω = 0.73 – 0.81) across the three waves.
Social withdrawal
Social withdrawal were evaluated by the Hikikomori Questionnaire [27]. This questionnaire comprised four items on social withdrawal symptoms: marked social isolation in one’s home, significant functional impairment due to social isolation, distress brought by social isolation, and duration of the symptoms. Hikikomori was diagnosed with marked social isolation at home of at least 6 months and significant functional impairment or distress brought by the social isolation. This scale has shown adequate psychometric properties in Hong Kong young adults [5].
Suicidality
The respondents were asked whether they had considered suicide, attempted suicide, or injured themselves intentionally in the past 12 months. Based on these binary measures, the present study evaluated the level of suicidality on a 4-point ordinal scale: 0 = non-suicidal; 1 = suicidal or deliberate self-harm without suicide attempt; 2 = both suicidal and deliberate self-harm without suicide attempt; and 3 = had suicide attempt.
Data analysis
The present sample showed minimal missing data (0.3% - 1.2%) in study/job impact and physical impact and no missing data in other variables. We first compared the demographic characteristics and study variables across the three waves using chi-square tests and ANOVA with post-hoc comparisons using Sidak corrections. Eta-squared denotes the effect size of the comparison with cutoff values of 0.01, 0.06, and 0.14 indicating small, moderate, and large effect sizes, respectively. Cutoff values for correlation coefficients were 0.10, 0.30, and 0.50 indicating small, moderate, and large effect sizes, respectively [28]. In the present study, statistical significance was primarily set at p < 0.01 level.
To model the inter-relationships among the 12 study variables (age, six items on situational impact, PHQ-4, meaning in life, societal pessimism, hikikomori, and suicidality), partial correlation networks were estimated in the 2021 – 2023 waves using mixed graphical model via the bootnet package in R [29].Age, situational impact, PHQ-4, meaning in life, and societal pessimism showed acceptable skewness (-0.07 – 1.07) and kurtosis (-0.92 – 0.85) and were modelled as continuous variables. The network models were estimated using the graphical least absolute shrinkage and selection operator (GLASSO) from the qgraph package to limit spurious edges [30]. The optimal model was selected based on the extended Bayesian information criterion. Tuning hyperparameter γ was set to 0.25 to filter out weaker edges to improve interpretability of the networks. All network analyses were conducted in R version 4.3.3.
Network stability was evaluated using 2,500 case-dropping bootstraps via the bootnet package [31]. The correlation stability coefficient (CS-coefficient) indicated the proportion of cases that could be removed from analysis while maintaining a correlation of at least r = 0.70 with values of 0.5 indicate good network stability. To summarize the network characteristics, expected influence was used to evaluate the cumulative influence of the nodes in the network, together with other centrality indices (strength, closeness, and betweenness). Nodes with high strength or expected influence were regarded as the central nodes in the network. The network models were visualized using the springlayout via the qgraph package [32]. Positive and negative edges were displayed in blue and red, respectively.
We focused on the bridge edges between suicidality and the risk factors. Accuracy of edge weights was assessed using 95% confidence intervals. The networktools package [33] was used to compute the bridge strength as the bridging power of the nodes with suicidality. The moderate sample sizes (Ns > 1200) in each wave provided adequate statistical power to estimate a 12-node network. Finally, network comparison was conducted with 1000 iterations using the NetworkComparisonTest package [34] on the network structure and global strength across gender (males and females), age groups (youths and young adults), and waves (2021 - 2023). We conducted invariance tests on centrality of individual nodes and edge weights. To adjust for multiple testing, invariance tests of edge weights were conducted at p < 0.001 level.