1.1 Study design and participants
In this cross sectional study, also the first data wave in a longitudinal study, of Chinese nursing students, we obtained permissions from three universities’ management, then provided information documents including study descriptions, guidance for completing the questionnaire, consent forms, and a QR code for the paperless questionnaires to student counselors, who then distributed these to students. At the same time, volunteers were recruited in each class to help with the promotion of the project. The voluntary nature of participation without any conflict of interest were emphasized to participants. The time needed for completing the questionnaire was 10~20 minutes. All participants provided written informed consent.
1.2 Measurements/Instruments
1.2.1 Depressive symptoms
Depressive symptoms were measured by the Depression Anxiety Stress Scale 21 (DASS 21, validated Chinese version) [27]. This 21-item DASS assessed three dimensions of mental health symptoms over the past week, including depressive symptoms with 7 items focus on low self-esteem, low mood and a poor outlook for the future. Each item was scored on a Likert scale ranging from 0 to 3. (0 = did not apply at all over the last week, 1= applied to some degree, or some of the time; 2 = applied a considerable degree, or a good part of time; 3 = applied very much or most of the time), with a significant cutoff score of ≥ 10 for depressive symptoms. The summing scores of depression was categorized into the 5 severity levels, i.e. normal (0-9), mild (10-13), moderate (14-20), severe (21-27), and extremely severe (28+) [28]. The instrument is frequently used for assessing medical students [29] and Cronbach’s alpha of the Chinese version is 0.83 for the depression subscale [30]. For this study, the Cronbach’s coefficients alpha is 0.89.
1.2.2 Individual, family and social risk factors
Each risk factor was chosen based on the criteria that there was an empirical basis in the literature validating the variable’s potential impact on depressive symptoms outcomes. Potential risk factors included gender(male/female), number of friends (<3 / 3~5 / >5), and household income(low/medium/high), academic performance(poor/medium/excellent), critical negative life events in the last year (changes in family [yes/no] / broken up in love [yes/no] / hospitalizing [yes/no] / examination failure [yes/no]).
We adopted a standardized questionnaire—Self-rating Questionnaire for Adolescent Problematic Mobile Phone Use (SQAPMPU) —found appropriate for evaluating PSU in adolescents [31]. SQAPMPU is focused on feelings or attitudes related to mobile phone use with each item describing a potential problem. It comprises 13 items in total, and cover three dimensions including withdrawal symptoms, craving and physical and mental health status—e.g., “I feel anxious if I have not checked for messages or switched on my mobile phone for some time”; “I need to spend more time on my mobile phone to feel satisfied”; “My productivity has decreased as a direct result of the time I spend on the mobile phone”. Responses to each item are on a five-point Likert scale (1 = not true at all, 2 = slightly true, 3 = moderately true, 4 = very true, and 5 = extremely true). The scores ranged between 13 and 65, with the 75th percentile (tell score here) used as the cutoff point.
The Chinese version of the Pittsburgh Sleep Quality Index (PSQI) [32] was used to assess the quality of sleep of university students in the last month.The PSQI consists of 19 entries covering sleep quality, time to fall asleep, sleep duration, sleep efficiency, sleep disorders, hypnotic medication use and daytime dysfunction. Each dimension is scored from 0 to 3. The total score ranges from 0 to 21, with a score of ≥8 indicating poor sleep quality [32]. The Cronbach alpha coefficient was 0.87 in the present study.
1.3 Statistical analysis
Data cleaning was performed before the data analysis, considering both range and consistence checks. A standard statistical package (SPSS 23 for windows, SPSS Inc.® headquarter, Chicago, USA) was used for analyses.
All of the risk factors were used in a prior study by our team [33].In this study, each of the eight risks variables was categorized into the high- and low-risk groups using the cut of scores. Among the eight discontinuous variables, household economic level as low, numbers of friends as less than 3, academic performance as poor, sleeping quality as bad, physical activity as low, PSU, having experienced hospitalization and breakup in the past year all were defined as having high risk. and the others as a low risk for each factor. Binary logistic regression was used to identify risk factors for depressive symptoms that were statistically significant.
Multiple risk analysis [34] was performed with the six factors that had a significant relationship with depressive symptoms to create a multiple risk score for the individual based on the total number of high-risk factors. This was done to produce a single, easily interpreted score that could group the participants for comparison on the outcome measure, i.e level of depressive symptoms. The scores of all high-risk factors were then summed to obtain a multiple risk score. Test for trend was used to ascertain the relation between the multiple risk classification and the depressive symptoms individually. Finally, a 2-step cluster analysis was used according to the Jaccard method [35] and χ2 tests was used to test the relation between the different clusters and the level of depressive symptoms.