2.1 Study population
This study used the publicly accessible data by the National School of Development of Peking University from the 2020 China Health and Retirement Longitudinal Study (CHARLS), which is an ongoing longitudinal investigation that focuses on middle-aged and elderly individuals in China [29]. Similar to the U.S. Health and Retirement Study (HRS), CHARLS has conducted an extensive survey of middle-aged and older Chinese residents aged 45 and older since 2011, providing data on age, health status, economic status, and social support. The study sample comprised households with members aged 45 years and older and employs a multi-stage stratified probability-proportional-to-size (PPS) sampling technique. The research team developed the CHARLS-GIS software package to establish village sampling frames. Trained investigators conducted data collection through in-person interviews. The data collected for CHARLS are managed by the Institute of Social Science Survey at Peking University and are publicly available on the CHARLS website. This sample involved 19,395 participants from 150 counties/districts and 450 villages/urban communities in 28 provinces [30]. The participants were middle-aged and older individuals, defined as those aged 45 years or older (middle-aged: 45 to 64 years old; elderly: 65 years old and above). Respondents with missing birth years were excluded. Figure 1 shows that 10,884 participants were retained after excluding those younger than 45 years and those with missing values for the main outcome variables.
The original CHARLS was approved by the Ethical Review Committee of Peking University. The IRB approval number for the main household survey, including anthropometrics, is IRB00001052-11015. All participants were asked to sign an informed consent form before the study commenced, and no ethical permission was required for the secondary analysis of CHARLS data.
2.2 Definition of Economic Situations
The CHARLS indicators on economic status are divided into household income and household consumption segments. The household income section covers respondents' income from work, public transfers received, work benefits, household income from agriculture, household income from self-employment or business, as well as household income from financial management and rental properties. The questions in the household income section questionnaire vary from one round of the survey to the next. Yuzhou Wang et al. from the official CHARLS team suggest using household consumption indicators rather than income to measure living standards [31]. They argue that consumption is a more reliable measure of long-term resources than current income, particularly in low-income rural areas where incomes fluctuate significantly from year to year. Moreover, consumption is considered to have a smaller margin of error in measurement compared to income [32]. We ranked the family consumption status of urban and rural participants separately, categorizing them into five categories from low to high (≤20%, 21-40%, 41-60%, 61-80%, >80%). The five categories are further divided into percentile ranks: Rural P20= 300, P40= 600, P60= 1000, P80= 2000, Urban P20= 400, P40= 800, P60= 1500, P80= 2750 (monetary unit: RMB).
2.3 Assessment of depressive symptoms
Depressive symptoms were assessed using the Centre for Epidemiologic Studies Depression Scale (CES-D-10) [33], which has been validated among older participants in China using CHARLS data [34, 35]. The CES-D-10 includes 10 questions with 4 response options: 1) None or almost none (less than 1 day); 2) Rare (1-2 days); 3) Frequent (3-4 days); 4) Almost always (5-7 days), reflecting the participant's experiences over the past week, such as feeling bothered, having trouble concentrating, feeling depressed, and feeling hopeful about the future [36]. The total score ranges from 0 to 30, with a score of 10 or above indicating the presence of depressive symptoms (0 = Normal, 1 = Depressive Symptoms). A cutoff score ≥ 10 was used to identify the respondents who had significant depressive symptoms [33, 37].
2.4 Measurement of determinants
In this study, the determinants of depressive symptoms among middle-aged and elderly residents in urban and rural areas include four key outcome measurements: depressive symptoms, demographic variables, health-related variables, and health behaviour variables. (Table S1).
Demographic variables include urban/rural areas, gender, age, marital status, and education level. In the context of urban/rural areas, the Hukou system in China serves as a population registration mechanism that designates an individual's residency status as either rural or urban. Specifically, individuals holding agricultural Hukou are categorized as rural residents, while those with non-agricultural or unified residence Hukou are classified as urban residents. The Hukou system, a longstanding feature of China's administrative framework, mandates that all Chinese citizens register within the system, designating their residency as either agricultural or non-agricultural (commonly referred to as rural or urban).
Health-related variables include self-reported health, satisfaction with their children, and life satisfaction. This category of variables can indicate the participants' physical and mental health status.
Health behaviour variables include chronic diseases, social activities, duration of sleep, alcohol consumption, and cigarette smoking. This group of variables can provide insight into the types of behaviours that may impact the health of participants and how frequently they occur.
Economy-related variables include medical insurance, endowment insurance and monthly family consumption per capita. These variables indicate the participants’ economic status. In China, "Medical Insurance" and "Endowment Insurance" are important components of the social security system. Medical insurance is a social security system established by the Chinese government. Its primary purpose is to ensure that citizens can receive appropriate medical treatment when they are sick or injured, thus reducing the financial burden caused by illness. Depending on the insured population, medical insurance can be divided into categories such as Basic Medical Insurance for Urban Employees, Basic Medical Insurance for Urban Residents, and New Rural Cooperative Medical Insurance. Endowment insurance, on the other hand, is to ensure that people have a certain source of income after retirement to sustain a basic standard of living. Furthermore, Basic Pension Insurance can be subdivided into Basic Pension Insurance for urban workers and Basic Pension Insurance for urban and rural residents.
2.5 Statistical analysis
All statistical analyses were performed using Stata 17.0 software (StataCorp LLC, Texas). First, the characteristics of the participants were presented as frequencies and percentages. The chi-square test was utilized to compare the differences in factors influencing the prevalence of depression among middle-aged and older adults in urban and rural areas. The variance inflation factor (VIF) tests for possible covariance between the variables. As shown in Table S2, all the values are less than 10. This indicates that there is no covariance between the variables, and the selection of the variables is meaningful [38]. Then, an adjusted log-binomial regression model was used to calculate the risk ratio (RR) of the covariates to analyze the correlates of depressive symptoms among urban and rural middle-aged and elderly individuals. Two-way interaction log-binomial models provide additional insight into the interaction factors that influence the occurrence of depressive symptoms based on varying household expenditures per capita in urban and rural areas.
The general form of the log-binomial model was:
P indicates the incidence of the event outcome, usually referred to as prevalence. Log-binomial models are a valuable tool for exploring associations between exposure factors and dichotomous outcomes (e.g., presence or absence of disease) in epidemiological studies [39]. This model is a specific type of Generalized Linear Model (GLM) used to analyze associations in cross-sectional studies. It directly estimates the prevalence ratio (PR) or relative risk (RR) rather than using the odds ratio (OR) to characterize the association between exposure and outcome. When the incidence of event outcomes is high (>10%), using OR to describe the strength of association will overestimate the association between exposure and outcome [40]. This approach can help prevent the issue of overestimating the OR. The log-binomial model for the categorical variables calculated relative risks (RR) with 95% CIs.