Study area and population
This study used data from the initial enrolment phase of Tehran University of Medical Sciences (TUMS) employees’ cohort study (TEC) in Tehran, Iran. This phase was carried out from the beginning of January 2018 to March 2021. In the cohort study, the sample consisted of the employees of TUMS and other institutes affiliated with this university who were willing to participate in the study (n = 4461). The TUMS Research Department formally informed all university-affiliated centers and faculties about the research. At the cohort center, data on the physical and mental health of the participants was collected (17). In the baseline phase of this cohort study examined 766 variables related to the health of the employees. In this present study used the variables such as demographic characteristics, socioeconomic status, educational status, and stress, anxiety, and depression disorders.
Study Variables
Depression, Anxiety, and Stress Scale (DASS-42): This scales measures depression, anxiety, and stress as proxies of mental health status. It is divided into three subscales: anxiety, depression, and stress, with 14 items in each. The items are graded on a four-point scale: never (score 0), rarely (score 1), occasionally (score 2), and always (score 3). The depression subscale assesses dysphoric mood, a lack of confidence, hopelessness, a sense of worthlessness in life, a lack of interest in involvement, a lack of enjoyment, and a lack of energy and strength. The anxiety subscale assesses physiological overarousal, fears, and situational anxieties. Finally, the stress subscale assesses the difficulty in achieving peace, as well as nervous tension, irritability, and restlessness. The tool's validity and reliability have been confirmed in the Iranian population (18), and the scale has frequently been used in studies carried out in Iran(19, 20). A higher score on this scale indicates that the individual suffers from anxiety disorders, depression, and stress to a greater extent.
Objective SES
Households' socioeconomic status (SES) was made up of components such as education, wealth index, and social class. Education was divided into eight categories (Illiterate, primary, intermediate, diploma, upper diploma, Bachelor of Science, Master of Science and Doctor of Philosophy).
The wealth index was calculated using data about durable assets (dishwasher, microwave, personal computer/laptop, washing machine, LCD/LED TV, DVD Players, home theater systems, access to Internet at home, automobile, total price of automobiles owned by family, number of rooms per person, and floor area per person). After that, the Categorical Principal Components Analysis (CATPCA) was used to calculate the household wealth index (21). It should be noted that higher wealth index scores indicate a higher level of economic well-being.
CATPCA was also used to assess participants' social class by collecting data on things like how frequently they went to music concerts, cinemas, theaters, or restaurants, how many national and international family trips they took, how much money they spent on internet, and how many extracurricular books they read. Higher social class scores indicate a higher level of social status.
Subjective social status (SSS)
The SSS mediating variable reflects an individual's perception of his/her (or his/her family's) socioeconomic status in comparison to other members of society (22, 23). In this study, the SSS of the participants was assessed using the MacArthur scale which was converted to a five-point Likert scale(9, 24). The participants were asked: Which of the five socioeconomic classes does your family belong to in today's Iranian society? (Upper class: score 1; Upper middle class: score 2; Middle class: score 3; Lower middle class: score 4, and Lower class: score 5). Individuals with higher SSS scores have a worse subjective social status.
Statistical Analyses
The CATPCA method determined that the first component that explained the greatest proportion of the variance in the composite variable was the best substitute for wealth and social class indices. In PCA technique, a large number of highly correlated variables are converted into a smaller set of uncorrelated variables (25). The variables "mental health, wealth index, and social class" and "education and SSS" were analyzed using "interval" and "ordinal" scales, respectively.
To assess the correlations between the research variables, the Pearson correlation coefficient test was used. The mediation analysis was then carried out using Anderson and Gerbing's two-step SEM (measurement model and structural model) approach (26). Using this method, the direct and indirect effects of SES indices on mental health were investigated while accounting for SSS's mediating role. In SEM, all parameters were estimated using Maximum Likelihood Estimation approach, and indicators of RMSEA < 0.05, SRMR < 0.08, CFI > 0.9, and TLI > 0.9 were used to test goodness of fit of the model (27). All SES indices were first incorporated into the mediation model as the composite SES-index (a combination of wealth index, social class, and education), and then they were inserted into the model separately by using STATA version 14.0 (STATA Corporation, College Station Texas) software.
SSS's mediating role was evaluated by calculating standardized path coefficients for indirect and total effects using the variance accounted for (VAF) formula (28).
Mediation Percentage (VAF) = (Indirect Effect/Total Effect)*100
VAF values > 80 % indicate full mediation; 20% < VAF values < 80 % indicate partial mediation, and VAF values < 20% indicate no mediation (29).
Ethical approval
for this study was obtained from the Ethics and Research committee of Tehran University of Medical Sciences (TUMS.VCR.REC.1398.829). Before registration, all participants read and sign the informed written consent form. A copy of the signed consent form is given to the participant. The guidelines on research involving the use of human subjects (beneficence, non-maleficence, veracity, confidentiality, and voluntarism) were strictly adhered to according to the Helsinki Declaration. Participants did not incur any cost by participating in this study and there was no financial inducement.